people somehow manage to break the ma box on relatively standard cars, so turbo/supercharger is of course going to prevent problems with power/torque increases. supercharging is much kinder to the transmission over turbo, as obviosuly the power comes in much smoother so there isnt that "bang" of power that comes in over a thousand rpm or so.
mechanical sympathy on gearboxes may save synchromeshes if you dont smash it in and out of gear, with lightning fast aggressive changes, particularily on downchange. but nothing stops the fact when applying full power and torque, the gears tear themselves apart - other than not using the power available - in which case there would be no point going turbo or supercharger in the first place and IMO a world not worth living in

lol
the BE4 gearbox conversion we sell is certainly a cheaper option than straight cut gearsets for the MA box, which often are not guaranteed for the power expected from turbo/supercharged 106/saxo - so we think its pretty good value.
people often overlook transmission when building a highly modified car, happy to spend thousands on the engine making the power (and rightly so, powers good

) but transmission upgrades are always a bit of a sting when you have to buy them. especailly when you start looking at professional motorsport gearboxes etc, where they are often double the price of the engine!
when i was running the MA box i never suffered a driveshaft failure (except from when an engine mount broke off the gearbox and pulled the shaft out, but cant count that one as a breakage)
they are pretty strong - within reason, examle: dont go bashing over rumble strips or over jumps at full boost and landing with your foot nailed to the floor! or piling on armfuls of lock and lighting the tyres up, theres a bit of mechanical sympathy/common sense on that one!
im not sure how standard shafts would get on with sticky drag slicks - so thats why we made our custom shafts to the specifications they are, to make sure they can!
- colin.