Hardcourt split is awfully harsh to Sampras IMO. Sampras is a top 3-4 fast court player of all time, plus he would be a big matchup issue for Nadal. Sampras was also no slouch on slow HC, won the AO twice and Miami 3 times. Sampras is IMO better on any hardcourt, fast, slow, medium, you name it.
Overall I think it would be
Clay - 11-0 Nadal
HC - (3 fast HC meetings and 3 slow) 5-1 Sampras
Todays Grass - 2-1 Sampras
So 13-7 . . . . .