My experience with heavies (isu in this case) being hit by sturm tiger is that the heavies stay crew shocked for a ridiculous amount of time.
Pretty sure this is a bug, but old crits I though removed are now popping up again when hit by ST shell. Annoyingly the AOE cause these crits to pop even on vehicles relatively distant.
Also for those saying 3 minutes, don't forget vet level requirements have been reduced, at vet 2 you start to pick up reload time reductions. Had a friend steal one of these yesterday and fire once though FoW at two tanks coming around a corner, it reached vet 2 immediately, was nice. Tanks were wasted, L2P issue clearly. Course how would he know where we were going to hide it?