NaB,
The point is that the USN assume that the ship can get to 30 kts, and that forms a starting point for the range of WODs that the aircraft design is built around.
As you so correctly point out, cat and trap design and operations involve a very close linkage between the two platforms. Available ship speed will drive the WOD available and thus maximum cat launch weights available on any given day, as well as the maximum recovery weights. More WOD also reduces speed to the wire, reducing wear and tear on the gear. Basically, ship speed is your friend for carrier ops, and the USN's devotion to that truth is, in my view, well founded. It's one of the drivers for nuclear propulsion.
I am a bit of a sceptic on the magic properties of EMALS, to be honest. It should deliver a smoother stroke, but peak loads on the nose leg are peak loads and given the short time available in the cat stroke cycle I'd expect any increase in launch weight to be low - but happy if I'm wrong on that. You also have to bear in mind that the immediate post-launch dynamics of aircraft are fairly critical, and ever faster speeds might not always be your friend there - in any case, plenty of tests required. Once more, very happy to be proved wrong.
The UK used to tolerate some sink on launch, with some sophisticated analysis on fly out profiles backing up that judgement. One of the interesting challenges of cat and trap will be taking USN led safety analyses and getting them through the new MAA regime. In my view, another reason why the ODH and DDH for carrier operation will have to be the FAA - you just can't split the ship/aircraft factors out here.
Best Regards as ever
Engines