I played with the KPS88 before going to the Prestige and they were in the mid-340s.
Fuji's comments are spot on.
On volleying, you just have to get the racquet to the ball and then apply the right amount of touch. The incoming speed of the ball against the mass gives you a nice deep shot with very little effort.
The problem is the serve - on the 380s. I serve my best in the low 360s. I can't accelerate the 380s as fast so I lose pace though I can hit nice spin serves with it. If I try to accelerate it too much, I have arm problems.
On groundstrokes, there's tons of power. Great for hitting heavy topspin. On stretch shots, you can flick the forehand and get reasonable depth with some power (Berdych does this regularly). On the backhand, you can get the ball back when stretched just by meeting the ball though it sets up a putaway for the other player. I can sometimes do a backhand flick (topspin or slice) on defense.
It's nice defending with a OHBH against high-kickers too as the mass takes the sting out of the ball on off-center hits and you can still get a decent ball back. I've been doing a lot more running and fitness work this summer than playing tennis as I'm training for a race. It really helps being in decent shape if you want to play with heavy racquets.
4 x IG Prestige MP, 70 cm, 376 grams, 386 SW, ALU Power @54