I haven't heard of anyone breaking the upper timing belt mount but I could see it happening. The lower front (A/C) mount is very important to holding the engine. It along with the lower front tranny mount and rear mount are the only things really keeping your motor in place when push the engine. The upper tranny and upper timing belt mounts are basically just there to keep the motor from falling out of the car.

Think of it this way, when you release the clutch to launch, your tires are trying to go forward without the car at first. If the suspension is stock and old, it will allow them to move slightly, then bind and get snapped back (this is known as wheel hop) like a rubber band. All the twisting motion is transmitted back through the axles into the engine where it's going to flex in it's mount. Without being completely supported, the engine is going to try to move on it's own too and if your missing a mount, your putting a lot of extra stress on the remaining ones, which they weren't designed to survive. In a situation like that, something has to give. Typically the tires break loose first and you simply spin them. If the tires have enough grip that they won't spin, the clutch might give and slip. If not, somethings going to break, typically axle's but I could see the mounts giving way, I haven't heard of too many doing that however but then again, most people run all 5 mounts. I would highly recommend obviously usnig that mount, even if you don't keep A/C.