I've got an Acratech GP-s and LOVE it!
It's literally one of my favorite pieces of gear! I most regularly use a 70-200 (2.8 IS) + extender + 5D/7D + grip and occasionally a flash. That's in the 8-9lb range (w/o flash) and it preforms flawlessly at every setting. More often than not, I do NOT lock the head down completely. I get it in that sweet spot where it will stay put, but I can still pretty easily adjust the lens.
I rented a 200mm f2L and a 1D4 (and extender) and used the head and it also performed pretty darn well, even in the gimbal position, but I didn't have much time with it. This is a bit heavier than the 300 mk II. I wouldn't try more than a 400 2.8 on the head, and I'm sure it'd hold steady, but the gimbal would be bit off as others have reported. I haven't had any problems with my head.
Well, I did have one…my panning knob actually got stuck once. Acratech is a short drive from home, and they took a look, replaced the entire panning base and some of the inside rubber all for free! They're an amazing company, and I love their products!
Not many people rent acratech heads, however, they loaned me their long lens head for a couple weeks for simply the cost of shipping. It was great with a 500mm f4.
Side note: The GP series of Acratech heads allow you to mount the clamp on the underside of the ball head, and put it on top of the panning base for panorama work. This is a very handy feature, and while a ball head isn't quite as pleasant to work with upside down (the knobs don't feel right to me), it actually works very well, and saves some extra gear.
If you do get an Acratech head, I strongly recommend paying a little more for the level clamp. It's really handy, gives me a little more confidence, and I think is quicker. (If you don't they usually let you trade in a regular clamp for the difference in price, and that's how I got my lever clamp since it's newer than my head.)