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
The MX5 5 speed gearbox is a delight having driven a sisters MX5 and my own M3W, which fun aside was the best feature of the car. A variety of sources confirm that torque handling on the 5 speed is nothing approaching the figures you suggest though Dave. I remember Griffo of this parish being concerned when his Ecoboost converted Plus 4 hit (from memory) 255 lbft on the dyno, given the generally accepted slightly lower max torque handling figure. The six speed MX5 box in direct contradiction will handle greater torque though but the accepted limit seems to be closer to 300 lbft.
Having said this the MX5/Miata forums suggest the six speed is a little less precise (it would be diffcult to improve on predecessor) and some even complain that the car so equipped lacks kick in (overdrive) 6th
