As sad as it is it's not all that unlikely that any gearset you get will have some issues at some point, especially with the power you're looking at. Some factors to consider:
- gearbox and clutch strength are measured in torque, not power - if you're making 700hpatw @ 7500rpm and that's peak then that's obviously more torque than someone with 700hpatw @ 8500rpm
- even given the above the ratings those manufacturers give are just estimates
- you'll need to pay for the box to get built too, not to mention any bearings+syncros that need replacing, plus the cost of oil and freight (it all adds up)
- you want a warranty and for it not to be near impossible to "claim" on it. there'll typically be two warranties too, one for the gearset and one for the gearbox build (i.e. build - if a bearing sh*ts itself, gearset - if a tooth breaks). so a local place with a good rep to both buy gearset through and do the build imo, one place and therefore not two companies blaming each other if something goes wrong
- the bearings, syncros and casing will be all standard (more or less), maybe even the output shaft (may depend on whether you replace 5th), and possibly some or all of the selector forks - any of those could fail, the casing may flex, there's still stuff that can go wrong
It might sound scarey but it's possible you'll have the gearbox for 10 years with one failure in there somewhere and a warranty claim, which given your power level would be a good run imo. Remember that even the big dollar holinger boxes have issues at times, otherwise people like tomei wouldn't stock spare parts