A friend (and engineer) has a MK!V running over 1500 HP and is running a stock belt and has never had a failure.
Way back ... I worked in the plant that made 60% of the timing belts in the world. I actually met the guy who invented the timing belt. That was way before my hair was grey.:icon_bigg I ended up running the life-cycle test lab before I left to go to school to be an engineer.
I tested many belts to failure and they usually go out because of broken teeth. Major places to check are the base of the teeth- even a slight crack or tear means it is already in a failure stage. Sometimes a jacket stitch failure would let the (tooth face) jacket strip back and teeth would shear. If a tooth jumps it will usually break off pretty much instantly and the other teeth also strip off. They all go together.
They are way over designed for the load applications and life-cycles recommended. I just inspected one with 120,000 miles on it (low performance daily driver) and decided not to change it (until I get time to get around to it). I don't recommend gong over about 60 or 80,000 miles unless you really know how to inspect one.
Tensioning is very important. Re-tension them occasionally for longer life (maybe at 30-40,000 miles). Of course if you are running above stock HP all the service times should be shorter.
