Finally found the pic I was looking for...
This pic shows a Tiger II that first was tracked with a well-aimed AP round towards the tracks and then knocked out with a 90 mm AP round by a M36 Hellcat (you can see the hatches blown open, I don't think the crew survived that one...).
From the looks of things, the tracks seem to have absorbed the impact energy of the first AP round, since the drive sprocket is largely intact, most likely with some seriously damage within the gearbox itself, since the Tiger II couldn't move afterwards.
This seems to confirm that welded on spare tracks may very well work against an AP round once, but after that it is just dead weight.
Cheers, Taskforce