People build 600hp cars all the time, out of smaller and older engines, and they last for years.
If you're not racing it the time it spends making 600hp is minimal. Can't really imagine anywhere on the road you could keep your foot up it for more than a few seconds before you either sh*t your pants, stack it or run out of road. Even 4-500 hp is a lot for a road going car
If your gearbox is rated at 500nm, that's likely to be at 100% of the time for 100's of thousands of kms. 750nm for a second or two here and there probably isn't going to grenade it immediately.