Biggest problem for Elite and the pimax is the need for parallel projection which is something of a 30+% performance hit alone, and the Index won't need that.I have always had reservations about the Pimax in terms of how well it would perform on games such as Elite and other more resource intensive games - as you say, anything other than those 20 minute wonders (which will likely work quite well) will have performance issues. My play time is generally not spent with those 20 minute wonder games.
There is little chance of enjoying Elite on the Valve Index at the120 or 144Hz modes, with the additional FOV it'll be interesting to see how it plays at 90Hz even. The Index has the same res my Vive Pro which is just on the cusp of mantaining 90FPS on my system (which is your spec) with Steam and in-game SS set to 100% and lots of settings on medium other than textures, adding the extra 20 degrees of FOV would likely tip rigs of our specs into reprojection an awful lot (meh). The Index's 80Hz mode may help with performance though....
I have an Index pre-ordered but I don't hold my hopes out for great performance in Elite, I guess time will tell! I'll likely keep hold of my Vive Pro and decide which I use for certain games. If the Index works well enough then I'll maybe consider selling my Pro + Wireless Adapter to recoup a bit of cash.
Also, I suspect the new faster hz also handles motion smoothing, but in stead will allow for rendering 60 real frames instead of 45 and that also shouldn't be a big difference.
But Valve is saying this is a premium set, so it will require a serious rig to run, no doubt.