RE: sail for 20-30 mph wind
Are you racing?
It sounds like it if you had a "RBC" specific (Race Board Class) sail.
Sounds like you simply must have cams.
I haven't done much racing of any kind lately, but toward the end of my "racing career" I was doing well with camless Sailworks Retro's on my longboards.
I had some RBC sails available, but found the Retro was overall faster and lighter.
You might try any of the following:
Sailworks Retro 7.5 (they even made a 7.5 m2 RBC Retro a few years back), Sailworks 7.2/7.8 m2 NX slm (4 cams), Severne Overdrive 7.5 m2, of perhaps the venerable NP V8.
I'm a bit suprised you are looking for a 7.5 m2 for 20-30 mph (17.4 -26.0 knots) as it would seem you'd be faiirly overpowered in 20 knots + with that large a rig, but you are the guy that has to hold it down, not me.
You might also want to take a look at the Aerotech Dagger which is a longboard specific sail, but not all X-ply.
Hope this helps,