I like the idea of the 6-speed box. But remember all you will be doing with that is changing gear more often. Looking at the photos of the 6-speed, I dont know how it can be stronger, even double the strength of the 5-speed. Why? Well for starters the gear teeth look somewhat small and straight cut are not as strong as helical for the same given size. Perhaps the theory of input shaft flex of the standard box has been addressed with the 6-speed, certainly has with the Bell upgrade where shaft size has gone up from 19 to 27mm on the 5th gear end. (undercut).
Have a looky at the picture a bit closer. Whats that large aluminium plate for? Looks to me like a strengthening plate for the rear thrust bearings, moreso for the output shaft where there is incredible forces pushing the shaft back against the bearing thrust plate. This force is of course offset with helical gears to an extent.
As for case flex ...... as much as it probably happens on high torque situations, I think its more of an issue coming from too much pub talk. Most flex will be in the lower gears and 5th where there is the greatest distance from the crownwheel/pinion contact to the 5th or 1st gear. Least 'flex' would occur in 4th gear. You certainly wont be able to measure this 'flex' with a ruler, nor see it happening. But to be open to comment, how many of you Renault box users have experiences 'case flex'?????
As for the pinion, well again, you abuse it, it will break. I sold a Renault 20 5-speed to a guy who has an off roader. He is on his 3rd crownwheel and pinion set. Why? Because when they go over jumps, they have their foot down and when in the air, the wheels are 'free-wheeling'. Come back on the ground and its instant power on through the box. Hence something tends to fail. Similar to revving the engine to max torque level, car with slicks, and then taking your foot off the clutch sideways - eg, instant take-up. I bet I could destroy a ZF or Porsche box the same way. If anything in there is going to break it would probably be the differential unit. Upgrade or change to a Quaife unit.
There are always going to be figures of strength tossed about in talk. To get a realistic idea - look at all those who have Renault boxs in their GT40s. Have a peek at the videos in 'other' site at guys hillclimbing and sprinting and they have no trouble with their boxes.
Sure, the Renualt box isnt going to all out as strong when you put Porsche, ZF and Renault side by side, open on a workbench.
If you do have concerns about its strength, then save your money and buy a Porsche.