Kev,
I have a Tornado which i have been running on and off for 8 years. I have changed to an Audi box now 016 type, ( mainly because i had two at silly prices) There is a bit more work in fitting this box as you have to get the chassis chanelled to miss the side cover bolts and also a little relief may be necessary to the rear crossmember to acomodate the rear cover.. NB This may not be necessary depending on your adapter plate thickness. Also you will need to get ndifferent drive shafts made to match the stud pattern of the box, and modify the gear linkage as the Audi box is a side changer. For all this, I have had no problems so far with the new box and although 1st gear is rediculously low (alright for crawling around car parks), I am soon to start racing it over here in Australia..... Conclusion, The Renault21 UNI is a fine box and easy to install if the car is designed for it but there is nothing wrong either with the Audi setup if done correctly. Good luck.
Shep.