The SL7 replica plug worked fine for me.

Has anyone else experienced any issues with the seat clamp? Mine isn't securely holding the seat rail at all. I've torqued it down to 9Nm, but I still feel a pinching force on the carbon rails, which doesn't seem right.
In the picture you can see the outside of the clamp only touches the rail on the top.

This clamp needs high torque. 12-15 NM are needed in my experience (own risk, I do not have official advise from VB). But you have to carefully align the rails with the clamp before. The clamps are full metal, so no worries here. Rails are fully carbon, so no big worries here, too.

I'm no expert in mechanics. But the expanding mechanism is obviously different.

More important is my practical experience with both. The one you posted, did not work with my bike (carbon paste and up to 10nm). The specialized replica just worked. I had good experience with the normal original specialized expander, so I went this route having in mind that some extra stability could not do any harm.

There may be other options that work fine too. Just reflecting on my own personal experience with my bike here.

Velobuild VB-R 268

Am I right, that you didn't consider the mandatory spacer? It's like the difference between specialized's SL6 and SL7/8. On paper the frames are more aggressive. But if you take the mandatory spacer into account, they are the same. So you have to add so m up the stack füre these frames!

I do not share your recap of this thread. Especially the saddle clamp (not the seatpost clamp) is no deal when done right. Through axles are a problem, that's true, and I'm still waiting for the replacement

While routing, I kinked one of the brake hoses close to the stem area and now I'm wondering if I should replace the whole hose. 

Happened to me, too. I replaced the hose. Brakes should be reliable at all times. On my second attempt it helped me to dismount the handlebar for routing the cables. The fork was hold by the top cap only.

I am almost finished with the assembly. Overall, I am very pleased with the frame. The workmanship is good as far as I can tell. I haven't ridden the bike yet, but overall it looks like the frame is stiff enough. At least it doesn't feel too wobbly in the shop. I will report back when I have ridden the bike.

Now I want to give a few more details. Tire clearance is no problem at all. The 30 Continental tires are 32mm wide on the rim, which fits perfectly.

The frame weighs almost exactly 1160 grams in size 58. The uncut fork weighs 424 grams. The cockpit weighs exactly 404 grams at 110x420 mm. The seatpost weighs 230 grams. Other accessories such as the headset topcap, headset, seatpost clamp and thru-axles add another 200 grams.

I built the frame with Ultegra Di2. The complete bike weighs 8.3 kg without the computer. That's not exceptionally light, but it's not terribly overweight either. The wheelset (Lightbycicle WR65 with Goldix 240SL) weighs 1550 grams.

The rear thru axle mount did not work for me. The thread was broken. The thru axle was also not mounted correctly. This means that after the paint job at the latest, it must have been noticed that the thru axle did not fit into the thread. Obviously nobody was interested in that. The frame was shipped anyway. This does not speak well for the quality control at Velobuild. I was promised a new thread for the thru axle. Unfortunately I have not received it yet.

The finish is purple chameleon matte. The quality of the paint is very poor. This is especially true for the durability. There is obviously no top coat. You only have to look at the paint once and there's already a flaw. I'm afraid it's not going to be very durable.

Then there's a minor problem with the top cap. It closes very flush when the handlebars are not tightened. But once you apply the correct torque, the top cap protrudes a bit to the side. This is mainly an aesthetic problem and doesn't look very good.

Now I have to make a few adjustments (like the saddle angle) before I can ride it outside. Also, the weather here in Germany is pretty bad at the moment. I also need new chainrings as the ones I have only work for 11-speed and the shifting performance is pretty bad.

Here are some pictures as a first impression. As soon as I'm outside and can take better pictures, I'll post them here.