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.
So in short what you want to say is Murray > Agassi and Sampras right
Interestingly, the only player to have beaten each of Big 3 and Murray in Grand Slam play is Tsonga, who happens to be the closest we have to a serve & volleyer among the top players today. And I'm pretty sure Sampras's game was a few notches above Tsonga's to put it mildly...