1) Playing in Doubles usually doesn't impair their Singles activity ; although, I strongly lowered the risk injury in Doubles for the next update.
2) I checked Sampras and there were a couple of issues :
- he wasn't strong enough from 1993 to 1995 (3 years)
- Junior players had too low Topspin, bringing it into their Pro carrier, and thus all top clay players were top fast surface players, stealing a few slams from Sampras
- the skill determination engine was overestimating the influence of the service, leaving Sampras with a bit too low rally skills
So I fixed all that, and now Sampras has a good chance to win 9 or 10 slams per career (although he got only 2 slams once).
3) So the main issue left here is that despite winning not enough slams, he always gets around 16-19 master 1000, while in real life he got only 11. Sampras always played his best in Slams (it was especially true at Wimbledon and the US Open). Agassi is a bit problematic as well. Both of them dominated a bit less than the Big 4, and thus it's harder to simulate their carrier accurately.
For McEnroe, after the few fixes, it might get a bit better, but maybe he needs to get a boost as well, but I can't find ranking points before 1990, so I can't do it.