Just a thought i had so bear with me. Although fed and nadal have since done it and it had been done before i think agassi's achievement may be greater due to the more vast differences in surfaces. The modern homogenising of the surfaces slowing grass and speeding up clay has meant that we have a case of "usual suspects" at every slam. Could nadal have coped with sampras, goran etc on super quick low bouncing grass? Laver did slam i believe with 3 of the 4 on grass (i might be wrong but i sure i heard that) Anyway thats my opinion.