On our outdoor astro track we have been snapping stock towers like crazy, with or without shimming out the tower. The only remedy was two carbon towers glued together. You can se some pics of our set up in this thread:
http://www.oople.com/forums/showthre...=102029&page=2
The same rear tower has been on the car since May last year, so a full outdoor and indoor season. The car is driven by my 7yr old son, and I can tell you that he really gets it up to speed before he crashes ;-)
The gearbox casing also seems to be really strong in this area, so no sign of breakage here either.
Also, we have no shims between tower and gear box, so I can really recommend this as a good solution to the problem.