Very interested to see more info come out on this one.  Looks more Epic WC rather than SC, more or less a typical horizontal shock, flex stay single pivot, with a tiny little rocker tucked up in the top tube.  Seems to be a traditional shock, I wonder if it can be flipped to have the lockout cable go forwards.  I'd guess it's in the ~40-60mm travel range with the shock nearly in-line with the seat stay and no leverage on the rocker.  Hopefully it's light enough to justify going less than 100mm travel.  From what I've read in reviews, a lot of people think the Epic WC and SC aren't light enough to justify against just getting an equally light 100-120mm bike with lockouts.

In my use case and preference, the FM909 has a pretty steep seat tube angle for XC racing.  I've got an FM936 and absolutely love it as an all-arounder.   But most of my XC races are fairly low elevation and aren't very gnarly.  The race I care about is smooth enough that I don't run a dropper, it's about 32 miles and 1800 ft ascent, lots of two-track, smooth singletrack, etc.  For those types of races, I keep my LCFS911 around, the only downside in my opinion compared to other name brand frames is weight, it's about 2100g in large (without shock), compared to 1300-1600g for the high end name brand frames out there.  When riding my FM936 hard on flat ground, I definitely feel like I'm right over the BB with a lot of weight on my hands, it's when I get into the steep stuff that the bike feels perfect.  If your races have lots of steep climbs and/or are chunky, the FM909 is probably perfect. 

Counterpoint, your Spur frame should be about 400-500g heavier than an FM909 frame.  Is 1 lb enough to make a difference in your races?  Do you have desire to keep the Spur as a 120/120mm downcountry bike with burlier tires while having the FM909 as a 100/100mm bike with XC race tires?  As 25 lbs, I'm guessing the Spur is already built up as light as possible, what do you really think you'd gain by building up an FM909?  Do the shock/fork on the Spur have ability to convert to lockouts?

Thanks. That's what I thought as well. Since you can't be fussing around with offset seatposts on a MTB and a dropper.

I'm 179cm with long torso and short legs (inseam of 81cm). I also have positive APE Index. I typically prefer 56cm road bikes and L sized MTBs. The Epic EVO L is definitely my size. I'm currently running the Voice Rhapsody VP4 on Large Size as well.

We couldn't be any more similar in our sizing lol.  I'm also 179cm, 82cm inseam, 185cm arm span.  I've got a large FM936 (identical geo except longer seat tube) with an 80mm -17deg stem.  I could see the preference for a shorter stem, but a medium would be way too short.  I believe there are set-back add-ons for droppers, but that would be the wrong choice in my opinion.  If you were to put a set-back post on a medium to have the saddle to bar distance be comfortable, you'd end up with a rearward weight balance.  So my opinion is that if you wanted a slacker seat tube angle, then you should probably be on a bike designed for one.  If you're okay with a steep seat tube angle, then a large is the right choice.

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It's meant for a 165x45mms hock

You can also use an FSA ACR headset.  Is there enough gap between the top of the compression plug and the top cap?  I thought mine had enough of a gap, but the top gap sinks down into the stem more than other top caps I've used, so it turned out to not be enough.  Added a few mm spacer on top and it tightened right up.

Not compatible.  You can just look at the hanger in the M04 picture and see that it's different.  UDH doesn't have the small screw next to the axle.  Therefore, SRAM transmission won't work.

You need shock hardware.  The 100mm linkage is 8x22.2mm and I believe the 120mm linkage is 8x30mm.