Minis never really had the legs for sustained high speed cruising, and the 5 speed gearboxes from Jack Knight aren't exactly bombproof, especially for the higher power motors.
BMW K1000 bike head conversion is a popular mod, and transforms the A-series into a true 100bhp per litre lump. It's quite cost effective too when compared with a port and flowed 5-port head, provided you do most of the conversion yourself.
I don't really agree with what you said about modern tuners, sure there some comedians out there, but the information and parts available today are light years ahead of what you could get years ago.