I just don't think you learn any more throttle control when running all that boost on a slower motor than just running a 17.5 without any boost. It's along the same lines of why driving 2wd buggy inherently makes you a better driver than running most other classes (I'm not going to say it, I'll just get yelled at...). Not to mention I think running all the boost is actually a lot harder to drive than having a linear power band, and all the speed at the end of the straight is pretty much useless. You want to be able to put the power down quick, like coming up to a big jump or squaring up out of a turn and getting back on the throttle. You notice this a lot more at places like RCHR or LIR, where the track is way tighter and you need the power out of the turns and jumps come out of nowhere.
Having said all that, I still think it's best for everyone to start out on something slower, like a 17.5/13.5, but I'd personally take a 10.5 without any boost over a 17.5 timed to the moon any day.
FWIW, I run a 7.5 without timing, geared conservatively.