Serve and volleyers can't compete with the Big 4 in this era. He'd do well because he's a great player but he would have no slams.
As for Agassi, he'd do better but he would still have no slams.
Sampras and Agassi zero slam?
Sampras level's in some of his slams runs was miles above:
- Murray USO 2012
- Federer Wimbly 2012
- Djokovic USO 2011
- Djokovic Wimbly 2011
- Nadal Wimbly 2012
- Del Po USO 2009
- Federer Wimbly 2009
Adding to that: His mental toughness was above all of them
He would have win many slams, for sure, not 14 though.
Agassi would have snatch some too.