yeah gearboxes are kinda weak. although when i blew mine my waterpump wasnt working correctly.
common problem is the waterpump weap hole leaking. simple fix. prices can range from 50 - 200 depending on what youre replacing.
i've only got about 500km on my slipper clutch so i cant really comment on how it helps. apart from that my gearbox still looks fine.
ive got heaps of parts but not really selling.
I can share my experiences with them if you like