SteamVR/Vive has full support for Async Time Warp (ATW) now, and they're moving towards Async Space Warp (ASW). ASW is definitely better, but it would be incorrect to wholly claim that the Vive is at fault here.
SteamVR also has a great option called "Always on Reprojection" - It can be used in games where your head is relatively static, i.e. non-roomscale games. It sacrifices a small amount of precision in head tracking to give your GPU ~2-3ms more time to render. When every frame needs to be done in 11ms, 2-3ms makes a huge difference.
You can use "Always on Reprojection" with ATW for an incredibly smooth experience through CPU/GPU spikes.
Today, if plug the Rift and the Vive into the same PC your game play experience is massively improved with the Rift headset.
Today, it's ASW that makes project cars so much more playable on the Rift.
Today, if he upgrades from a 1080 to a Titan X (ignoring other issues that I think could be resolved by a reformat) the gains aren't likely to be that noticeable.
Tomorrow however...
When ASW is developed and release for the Vive it will make it a level playing field and his Titan X purchase wouldn't have been necessary.
Tomorrow, the improved lighthouse base stations will be out and we might see some subtle tracking improvements on an already great system.
Tomorrow, the replacement headband with integrated headphones and wireless solutions for the Vive will make it a simply awesome bit of kit.
Tomorrow, SLI finally becoming viable with VR will mean that again he would have saved some money buy sticking with the 1080 and running that in SLI rather than two Titan X's.
I realise I'm coming off as totally standoffish, but I have no loyalty to either headset. I own both and he wants to improve his Project Cars experience today then he should buy a Rift. I'm not claiming the Vive is at fault (I use mine more than my Rift) but it's a simple fact that currently ASW makes the gameplay experience for games like Project Car much smoother and a much more enjoyable experience.
If he is to wait for ASW for the Vive (which there is no confirmed date of release) then he shouldn't buy a graphics card.
EDIT: Double checked, still no timeframe for ASW for Vive.
I should also say that while ASW is great, it does cause some artifacting. While this can go unnoticed in most games (like Project Cars) it does make some unplayable.
DCS benefits massively from ASW, but the new spitfire that was launched has massive issues with its gyro gunsight and ASW. ASW gets massively confused and makes it so distracting that it's essentially unusable. It's not a silver bullet.