What they all said...
...it's kind of difficult to talk about hitting 70%, or whatever, of your "hardest", because everybody probably has a different idea of what "hardest" means. If you're talking pure pace, very few guys except the ATP and WTA guys and gals hit 95% of their max pace, all the time.
But consider this: there is an article out there on the web somewhere where somebody figured that one of the keys to Sampras' game was that he had, shot for shot, a higher rotation on the ball than anybody else in the game. Sampras hit (and still does) an incredibly heavy ball that had pace and all kinds of spin on it. Federer is much the same. His forehand has lots of pace, but it goes in a bunch, too, because he's got a ton of topspin, too. It's not as evident as Nadal's heavy top, but it's there.
So in a lot of respects, you swing just as hard at a heavily topped ball as you do at a flat ball...it's just a different swing path, and one that is going to produce a ball path that'll give a higher percentage shot...high over the net, then dipping down into the court...