Glojo,
The reason for 30 knots (or thereabouts) is that it's the USN default assumption which drives the design of carrier borne aircraft like F-35C and indeed Super Hornet. It affects aircraft design for both launch and recovery, especially at the higher weights that are of such interest these days. Not to mention high temperatures.
I have friends who flew fixed wing off both Hermes and Ark Royal - and they all tell me that these were marginal ships with some really narrow safety margins for launch and recovery. A Sea Vixen pilot told me that they used actual aircrew weights (down to the pound) instead of a standard weight to wring out more performance. It's my view that a 28 knot ship would be a slow F-35 ship, but I hope I'm wrong.
The basic thing to remember is that cat and trap demands a very close integration between all aspects of the ship and the aircraft to achieve effective launch and recovery. That's not a criticism, just a fact. STOVL is more flexible, but you pay a performance price, and, it seems, an aircraft price.
LO - thanks for flagging up the selective acquisition report - good spot.
Best Regards
Engines