We all pretty much taper like that - it's not a matter of "WGA setup" - but if it's actually off from normal, then maybe it's having some effect. The clutch slippage is one reason why the traps may be lower compared to the mph. If it was slipping through the traps, you'd have an artificially low mph.

Doubtful it was the tune unless you were just knocking so terribly at the top of 4that it killed your mph. However, that wouldn't explain why you're bouncing off the rev limiter with such a low mph...