the difference between 260 and 300 I really big. I have boards that I rode at 270 that are nothing but frustrating at 300 (dont forget that wet neoprene can add a significant amount). Im also 6’4’’ and a size thirteen shoe which makes a difference on the pressure on the rail. No doubt if the guy is a decent surfer 4’’ and boxy might not have incredible performance, but truth be told most three hundred pounders just wanna catch waves and have fun, but then again most everybody does
“just wanna catch waves, stand up, and have fun”…so TRUE!
Besides, at 135lbs., I have ridden several 6’ x 22" wide boards thicker than 3.75" and they were just fine.
My first 8’7" gun at 19" was 4" thick.
My second, about 10 boards later, was actually THICKER, but before I really got into big waves with it, I thinned it to 3" thick.
“sinking the rail problem”? I don’t think so, a 300 lbs’er has lots of rail digging weight.
Of course, any thick board NEEDS hard turned down edges for penetration, and flat decks for precise feel of the banking needed, and rebound release if it starts to catch.
All those things have to go hand in hand.
As for carving…current windsurf Formula boards average 8’3" long, 38" wide, with about a 24" tail, and I can bank them into a rail set carve just like I can a 21" wide board, and that done at 25 mph speed!
You learn to ride what ya gots.