The 17.5 class at our track is about 2 laps down from nitro mod. What I'm noticing is the guys that have been doing it for longer know how to push a 17.5 to death with killer speed controls and tricks to get the motor to run cooler at the start of the race. Now add high dollar lipos and you have a class that would be cheaper to run with a more powerful motor that runs cooler.
However, the upside is that it limits the speed of beginners and lessens the carnage associated with beginners with lots of power. So, it's not without benefits.
I do think there should be more narrow rules in stock classes limiting what an esc can do and motors with non-adjustable timing. It wouldn't hurt to limit lipos to an n-1 scenario where n = latest and greatest lipo - 1 year. Maybe a mah and C rating limit. It looks like the technology is improving so fast that it's hard to really pin down any rules. Bushless and Lipo has been around long enough now though to nail something down, IMHO.