Oh dawn you guys have been exchanging a lot this night !
So, I successfully compiled the 4.15 Vrworks branch yesterday, added a few tweaks here and there (remove the black bars of the VR Preview for example), and started migrating our project template. So far I have absolutely no problems, performance increase is roughly the same as with 4.14 (ie. between 1ms and 4ms of frametime on our template scene featuring a few resource intensive assets).
Here’s what it looks like in the steamVR debug :
Now I’ve been trying multiple configurations in the render settings, and so far enabling LMS and SinglePass Stereo is giving me the best results.
I’ll try to bench a bit on one of our massive models today.
Also, if David C. from Nvidia sees this thread, thanks a lot again for the heads up ! 
EDIT : I’ll be on the Unreal Slackers discord if you want to talk more on the matter !