Sampras would be better than Federer on 90s style grass without question, and Federer would have alot of strong competition there besides Pete. So probably only 1 or 2 Wimbledons vs the 7 he has now.
U.S Open would be between him and Pete with some other contenders. Maybe 2 or 3 there.
Australian Open he would have alot of competition from Agassi. Probably 2 or 3 titles there.
French there was a ton more depth on clay then but no clay courters near Nadals level (but many of them around Federers level which is obviously light years below Nadals) so 1 or 2 titles there.
On average probably something like 8 vs the 16 he has now.