I had a T5 in my Chimaera, TVR used the best 'Z' spec needle roller bearing version originally developed for the most powerful hot Mustang in the mid 80's, don't get me wrong it's a good box but it's not a patch on the Mazda 5 speed in my Duratec Plus 4 which is an absolute delight to use.
From what I hear the MX5 five speed is perfectly reliable to 400hp and more importantly 350 lbs/ft, so re the Rover V8 application you could put one in a mighty five litre TVR Griffith without fear of failure.
The six speed Mazda box is a lot weaker and not as nice to use than the five speed, so having used them all personally I'd leapfrog the venerable old T5 jump straight to a Mazda 5 speed, it really is that good and way stronger than you might expect given the MX5 origins.
Best of all you can pick a good used one up for as little as £250 so while the Vitesse RV8 bellhousing might be expensive, it's worth costing the whole project out and keeping in mind if the Mazda box did fail its cheap as chips to replace, plentiful too
