"• First question : Would a 10k sata disk be fast enough to drive those ± 4 layers of video + 2 masking layers for each machine ?"

Don't forget that if you are using dual outputs as a opposed to the panorama option that "Use Layer" is the most efficient way to go.

That means that if you want the same clip on Output 2 as Output 1 you can "Use Layer" the number of the layer you are playing the clip back on for Output 1. (Hope that made sense!)

This should cut down your disk overheads considerably if you only need 2 layers of video and 1 x masking layer.

In my experience you would struggle getting 4 layers smooth of one Raptor.

Cheers

Toby