I'd be dubious that you could get more than 70bhp without ram-air effect. It seems to me that there's a definite ceiling around the 65-66bhp mark before costs become absurd (unless you have free labour).
Far more importantly, I think the useability of the engine diminishes as you eek the last couple of bhp out of it. Off-idle fuelling and an asthmatic bottom end seem to be the results of overtuning.