Carbon seatpost for MTB is so 2016!! 

Just kidding  I think you just have not been lucky. As mentioned, if you feel this is too close to the limit pick a longer seatpost. Then, seriously, I'll never use carbon seatpost on a MTB anymore. I use a RockShox Reverb as this is the best upgrade to a MTB.

Then, I still use a Carbon seatpost on my road bike and this is fine (a generic one). I don't land like a fat cow on my saddle, I lift my ass over bumps, for my comfort more then for my seatpost though 

So, just go for it

It's a tough call for you... I would pick 28 mm for Northern France/Belgium. Not sure tubeless or tubular (both are good options). The reason I selected tubular for my carbon wheels is really because I wanted a very light set of wheels, for the nice season, for the mountain (with 60 minutes climbs). Tubular themselves are not lighter then tubeless tires but the rims are. My rims are 300g, tubeless/clincher rims are closer to 400-420g.
I selected 23 mm tubular because they provide good comfort and light weight, but I'll try 25 mm tubular next (with a 20-30g/tubular weight penalty) for even greater comfort.

If you think about repair/setup pain, I think tubular is better. I have some latex in my tubular so I should be good for small punctures. Replacing a tubular on the road is fairly easy. The only pain is that it takes several days to put a tubular on when the rims are new (then just 24 hours of drying time when you replace a new tubular). Tubeless are as easy for small punctures, but they are a big pain when sealant doesn't work. Tubeless tires are difficult to mount, potentially impossible when stressed, under the rain, frozen, on the side of a dirty road. I don't want to face the day when sealant doesn't work on my road bike or MTB (I have used "mèches" once successfully though, no need to demount the tire). Removing and replacing a tubeless tire is one big pain.

PS: I agree that past 70 km/h is scary. It requires full focus on what's going on. My bike was still very stable at that speed and that's because it's a MTB frame, with a longer wheel base. The phenomenon you mention is called "guidonnage" in French. You can Google this key word for advices to fix the problem.

Well, I had most of the components for a long time (except the spokes) but I had to wait for my MTB wheels to be build to free the hub! Till then I was using ZTR Crest rims on XT hubs (my former 29" wheels). Those are good wheels, below 1.7 kg. I've been using those all through the winter with Schwalbe Pro One tubeless tires, 28 mm. It makes a really good set of wheels. On winter dirty, slippy roads, they've been fantastic.

Now the Carbon wheels 
- Rims: carbon, 30 mm deep, 23 mm wide, specific for tubular (a lot lighter then for tires). 300g each. From Peter who made them special for me w/o brake surface, all shiny, so beautiful! Very affordable product!
http://www.xmcarbonspeed.com/Productinfo.asp?f=1470
- Hubs: Novatec D711/712SB. Those are the hubs that were mounted on the 27.5+ wheels I bought from Peter. Cost is zero!
- Spokes: I gave a try to flat spokes, CX-Ray. They are expensive, even though Peter got a good price for me (more expensive then the rims...). I don't regret it, they are very light and look nice.
Overall, 604g front, 737g rear. It could be lighter with fewer spokes but my hubs were 32 holes, I thought it would be stupid to buy new hubs just to reduce spoke counts. But if you pick 28/24 combo you can save another 40-50g.

Tubulars are Vittoria Corsa CX III, 23 mm. I was a bit worried to get down from 28 mm to 23, but I have no regret so far. I wanted these wheels to be as light as reasonably possible but they also prove to be of very similar comfort to tubeless 28 mm (a lot better then regular tire/tube). Gluing process is scary at first, but honestly there is nothing difficult. I carry a light and compact tubular in my saddle bag, in case the latex doesn't work. Changing the tubular is actually faster then putting a tube in my tubeless wheel.

As you might have seen on Strava, I got more confidence today and topped 71 km/h in a descent, but I was very careful, light on the brakes, paying attention to any suspect noise. And I trust that these nice wheels made my climb a bit less painful. 10%, 10 km climb

And the bike is as finalized as can be 

I've build my carbon wheels. Not even tested yet but you can already enjoy some pictures! You can also see the new bottle cage (carbon with a painting that fits my bike - not custom painted though) from Aliexpress. I finally find that the red line on the tires is too much. I'll probably won't repeat when they are worn out. They won't last till next year anyway (the bike already has 3000+ km, mileage goes fast on road bikes!).

And the bike as pictured is 7.5 kg. This includes pedals, bottle cage, latex in the tires and... a pump (hidden in the seatpost). My spreadsheet tells me 7235g w/o pedals, bottle cage and pump. That's most likely light enough for what I will do. And it's a disc brake bike, which is always a bit heavier, and don't forget it's a MTB 

Pictures now!

- prefer light weight over aero, maybe 35mm profile max
- going to put 25mm tires on it and hoping for wider rims
- i am a bit worried about braking performance on carbon rim though, especially on long descent

- It depends on your roads. I live in the Alps, so I've picked light (30 mm) over aero. First, light helps more then aero in a 12 km climb, then aero is scary in a 12 km descent at 90 km/h. Any cross-wind and it's ugly. If you live in a flat area, pick aero! 35 mm sounds a good light/aero value.
- 25 mm seems to be a good compromise between weight and comfort. I get 28 mm tubeless aluminum winter wheels and 23 mm tubular carbon mountain wheels. If I was to pick one universal wheelset, I'd select 25 mm tubeless.
- I'm a poor rider but I get close to the best riders in my area on the descents thanks to my disc brakes. And "best riders in my area" mean Pro riders (like Romain Bardet), or at least people you see in the magazines, not just a good club rider. I understand that going disc means a new bike, but I would not consider buying expensive wheels until moving to disc road bike.

I like the Assos-like pose of the model
But I agree that their product seem to be of a higher standard than other vendors. I was specifically interested with some of their shoes, really nice looking, but I really don't need shoes for now 

I have just converted from 135x10 to 148x12 (the big jump!), and the good thing is that if you keep the same hub model you don't have to change the spokes. That's a nice saving.

Even from 135 to 148 mm, the spokes length was still perfect. But keep the hub exactly the same reference (apart from 142 dimension). I was lucky enough that my Novatec hubs exist in both 135 and 148 mm dimension.

A quick update here. I want to mention that Peter has replaced (free of charge) the rear axle that was too long. He has slipped it in an order I had placed recently (Spokes for my road bike carbon wheels), so that even the shipping was kind of free. The new axle fits nicely, with no shim, and is 24g lighter then the previous axle+shims.

This is a good example of high quality of service from CarbonSpeed!