Oh, pshaw, Krusty! The former P-40B couldn't catch bombers either, but I've shot down B-17s above the cloud layer (running the same unrealistic full throttle you mentioned) in AH. Was a long chase, but it can be done. Same goes for the A6M2. The Boomerang has a semi-decent climb rate (compared to the P-40) and much better firepower. It did shoot down just as many planes as the uber Ta 152, as well as the uber Meteor the squeakers are probably voting for.
People want this plane because it looks like it would be fun to fly. Small, turny and packing lethal firepower. Plus, it's an airframe from a country not yet represented in the game. Who cares about historical significance in here anyway? The historic early Spits, Hurris and 109s from "the Battle of Britain", as well as the early P-40s of "Flying Tigers" fame are rare birds; compared to the uber Rainbow XVI, 109-Gay4 and Pee 51D. Most sorties are in late war monsters anyway, so whatever is added will likely be seldom seen. Compared to the Ki-43's anemic firepower and legendary fragility, I'd rather take my chances in the Boomerang and would bet most others would too.