Devious--
The floats are permanenely attached, unless damaged
And yes they carried a considerable performance penalty. The A6M2 with floats could only do around 280 MPH, as opposed to about 325 for a normal Zero. This is still fast enough to be effective as a local defence fighter though. I think the N1K1 did around 310 MPH.
Sixpence--
Good questions, for which there are IMO good answers
I believe the ports in AH weren't intended to be used as airfields--I think they were intended to be an asset meant to be defended, not an offensive location.
However, some of the ports on some maps tend to be far removed from friendly airfields. This makes defence difficult. There's been quite a few times I sat in a port, only to watch a small group of enemies capture it before defence fighters could arrive. I feel ports could use some additional defence ability, because of their strategic importance.
Why float fighters?
If you enable land-based fighters at the ports, you turn the ports into offensive locations. Heavy fighters or such could take off from a port to attack enemy fleets/fields. This is contrary to what I think HTC wants the ports to be. Even if you found a way to disable ordnance from the ports, you'd still be able to launch fighters that are every bit as good (and possibly better) than what the enemy can likely muster--which would still disrupt the balance too much IMO.
However, a seaplane fighter cannot carry ordnance, and nor is it a true match for what the enemy will be able to bring. It'd be useful strictly for things like attacking heavy fighters and hunting for C-47's. In other words, a float fighter might be enough to buy some time for the real defenders to arrive. It'd give the defending side a fighting chance
J_A_B