Sampras would have been at or near the top of the rankings with multiple slam titles in any era. A great serve is a great serve, and his was arguably the best ever. To go with that, he was athletic, moved well, and by all accounts worked very hard on his game. Taking all that together, I have little doubt that he would have developed his game as necessary to compete in whatever era he was playing in.
I don't understand why, when asking how Sampras would do today, people just transpose the serve-and-volleying Sampras to the modern game. There's no reason to believe he wouldn't have developed his game accordingly based on the conditions and style of tennis being played...since that is exactly what he did in his own era. The guy could hold his own from the baseline with the best players of his generation. With his athleticism and work ethic (and serve), I have little doubt that he would have been a force in today's game with a modern, baseline-oriented playing style.