I ran 20 - 22 psi boost on my 1.6 rst, had a large intercooler, uprated rad, twin cooling fans, bigger turbo, ap clutch and pedal upgrade and a chip, oh and mark shead's fuel reg modification. and a STANDARD gearbox. I drove it on the track many times as fast as it would go. it was always driven hard. never had an issue with the gearbox. I had a spare one just in case. would have that car back if I ever found it again.
only things with tuning the 1.6 is once you fix one area of issue another soon appears, worth doing the whole upgrade in one go. oh and if you fit a bigger intercooler and radiator you will need to look at the alternator pulley, it will sit too close to the hoses and soon eat them lol ...