I tried aggressive power braking, and there was NO change. The car will start to spin the rear tires with the fronts locked, so there's obviously a point of diminishing returns. I usually load the engine to something like 1000-1200rpm against the brake and then nail it, that seems to work pretty well.

My launch seems to be the problem, due to the decent trap speed - which is why I'm guessing the issue is with low RPM power, not top end (also top speed isn't a problem, if the engine was weak on peak power, top speed would be a problem!) I can schedule a dyno run at the local speed shop for $75, maybe I can squeeze that in next Friday?