my standard t5 rebuild box lasted 5 years of over 400 bhp .then another 2 years of over 500bhp ,before being rebuilt when it did not need it ,
turned out clutch cover had gone week ,snapped eventually
diff did the same as is still going as we speak
imo standard good condition items or rebuilt to standard spec fine
everytime my saff went out .got its head smashed in most days ,so was well tested ,maybe i was just lucky
up to you pal just my opinion