The mainshaft is the weaklink once you put in a steel 3/4 shift fork. If you launch hard on slicks beyond 600rwhp you are on borrowed time.
That being said a customer of mine never broke a stg2 T56 I built with basic upgrades. Mellow launch and 9.20s@162mph in a 3000lb car. Gts76 Turbo ls3 rx7.
Supposedly at 3500lbs the Mainshaft (output shaft) limit for repeated abuse is about 600rwhp/tq on slicks. An upgraded viper 30 spline shaft is reliable to about 900rwtq... Beyond that the gears are the weak link. Especially 5th in top speed high hp racing.
There are a bunch of twin turbo Vipers running stock T56s into the 8s. How long they last is a roll of the dice.
If you have the $.... A T56 magnum is a very nice shifting transmission. I'm putting over 1100rwhp thru mine.
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Project: 1969 Chevelle, alum 5.3, GTS76 turbo, Chiseled Perf A/W intercooler, Holley Dominator EFI, Davis TC, JRi coilovers, 14" brakes, T56 Magnum. CTS-V Recaro seats!
Coffey Fabrication Nashville, TN doing the fabwork! 800rwhp+/25mpg+/9's@150mph goal 
https://lateral-g.net/forums/showthread.php4?t=52976
1993 Mazda FD RX-7, built LS7, T56 Magnum, Holley HP, Davis Tech TC, 8.8" 3.73 Cobra IRS diff, DSS axles, Ohlins coilovers, big brakes.
CCW 18x11 front 315/30/18's and 18x12.5 rear on 335/30/18 RA1's, 2850lbs
10.31@137mph 165mph Shift-Sector 1/2 mile speed
441ci AllPro headed motor and 200lb weigh reduction in the works....
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