Hmmm... J2m3... 1850 hp, 208sf of wing area, 7000 pounds, roughly, four kanonen... Compelling. I' m seeing differing data on the altitude performance. It looks like a ceiling of 37500 but at least the -4 model had unresolved turbo supercharger issues.
But, hell yeah, bring it. That wing and power loading make it look sort of like an interesting tweener. It doesn't set any speed records but has a damn fine climb rate and likely has decent turn performance. I'm picturing nik2 class turn and better climb.
Iirc from brief comparisons, if the J2M5 were added to the game it would outclimb even the K-4. It's been awhile since I've looked anything up on, but there are elaborate threads in the wishlist with lots of info on it. It has been in the wishlist for 10yrs or so.
Collectively they were built in small numbers, but not as small as some of the other ftrs already in the game, the J2M was built for a purpose/B-29 intercept, a purpose which it pursued with only limited success, however when engaged by ftrs it proved to be a worthy opponent. It had design flaws just like many of the other ftrs in the game, I believe it would see as much usage as other lower tier mainstay ftrs in the game, the 4-cannon might need a perk I dunno, the N1K doesn't have a perk...It was land based only iirc, so maybe a small perk on the 5.
With its ability to climb it might give the runners something to consider, top speed as you said is not up there with the ubers, but convert alt to speed...
