When I go above 75% swing speed I slice like crazy.
Telling myself to throttle down is a challenge at each drive. Your thinking about the 4 or 5 things you want to make sure are right; grip, stance, balance, spine angle, and tempo. The other 200 things that I should be doing I can't worry about right now :)
With that said, I hit about 220 to 240 at 75% and have a ton of control. The problem is on the course; you always have dog legs, elevation changes, narrow fairways, and don't leave out weather... so I find myself thinking... "I gotta hit this one harder to make this work.." bam... slice 40 yds into the desert.... oh I got 250 yds down range... but I'm dodging rattle snakes looking for a $2 ball.
Yesterday I went 75% on a hard dog leg right par 4 and that HiBore driver and that HX Hot Bite ball earned every bit of R&D; money spent on them by the manufacturers.... ball went 300+ yards and I found myself 20 yards from pin, just off the elevated green. I spent the rest of the round trying to duplicate what I did... yeah right.... so I know the distance is in there at 75% but I need to now work on duplicating the body coil, the uncoil, and the rotation of wrists during impact that I "obviously" got right on that one swing at 75%.