personally vibram, c4, stealth, tax, and any other rubber put on shoes made by respectable companies is all relative and shouldnt be the reason you climb any letter or number grade harder, if it is it is more mental than the rubber in my opinion
I for one have never had a foot slip and immediately think "damn I know i would have sent if i had ________ rubber instead of _________ rubber" ......no most the time i fall and/or my foot slips (which does not happen often because climbing shoe rubber is a thing of miracle and none of the rubbers that I have used have ever just slipped off any type of rock often) i think "f*** that was a bad foot" or "sh*t that move was hard as balls"