...and the Short Sunderland or the N1K1 wouldn't be a pushover either - but these planes were generally slower. Others, like the Catalina, AR-196, Walrus or the (Grumman) Duck could be really useful in real life - especially when flown far away from enemy fighters. Unfortunately one can fly relatively long distances in AH - which means that these planes become impractical in the game, unless we can find a way to make having these guys worthwhile.
Sunderland and N1K1 would be, in my opinion, pushovers.
H8K2 Emily is better armed, better protected, faster and has a better climb rate than the Sunderland. Its climb rate is, in fact, about twice that of the four engined aircraft we have in AH now, and its top speed is close to the same.
Sadly, it would be a monstrously large task for HTC to make it so I cannot think of it as anything other than an extreme longshot to ever be added.