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One time we rappelled into Englestead Hollow 300' on Imlay 8mm rope with micro racks. It was fine, but you did have to try and keep your speed down. All our rappel devices got really hot. A friend's Pirana flopped onto his bare arm after he got off rope and he ended up with a Pirana-shaped brand.

My wife and I use BMS microracks for canyoneering all the time. I think it is a great device for 9 and 8mm rope. Dave is right that the friction is unadjustable once you engage the hyperbar, but the hyperbar is your maximum friction available anyway. Anything more would be like ttrying to take friction to "11". One way I've found to take the friction to "11" or at least to "10.5" is to use both hands on the brake. The right hand in the traditional place, and the left hand above it on the rope as it feeds up into the hyperbar. I find this brake hand configuration makes things feel more secure when on wet, skinny rope.

I am surprised that canyoneers almost exclusively favor aluminum rappel devices. If I used aluminum, I would be replacing my descender at least every year, and I only do a modest amount of canyoneering. Some of the stainless steel bars on my microrack are 10 years old.

If you use an 8mm rope be sure to use plenty of padding, Especially over multi drops. The T strength of the rope is plenty adaquate im sure but smaller ropes can be a bit less forgiving on sharp edges. The elongation is typically worse and there is less surface area to absorbe damage. Also if using any rack use as many bars as your weight allows and take the drop slow as also less diameter is less heat disipation. Other than that the 8mm rope has a great weight advantage.