I use a 8.5's in both my RC10 Stealth and the B2.
Got a Losi JRX with the retro-fit XX box on it with a 10.5 and I've given it some with that - handles it no problem. I don't think you'd have any problem with an 8.5 in a XX. That's with Lipo too.
I have run brushed but whilst it feels great and it's good to be period, with the lack of decent cells forcing me to run Lipo instead of Nimh it does take more out of the motor.
I will run brushed in my more 'show' stuff that I will drive for fun but when 'racing' vintage it'll definitely be brushless.
Heard stories that a Hydradrive wouldn't be up to modern power though so wouldn't have one of those on (just in case you have one).
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