"Weak" is a bit subjective.
If you've got a stock or mildly modded XR2 and you're doing auto-x, you're not really going to reach the limits of the trans.
If you've got a giant turbo and standalone, pushing 20psi and launching at the drag strip, hell yeah it's going to break.
There's two weak spots in it:
-The differential. It's an open diff. It does NOT like one-wheel wonders, it does not get proper lubrication when one wheel is spinning way out of control. The causes the spider gears to gall to the carrier shaft and eventually shear the carrier rollpin, and the diff explodes. You can kill an XR2 diff with a bone stock car if you sit around and do burnouts all day.
-The gearsets. They're thin, they're not made for big power. Not an issue until you reach the 250-300whp range. Not an issue for a stock car or even a heavily modded XR2, it would take a pretty good build for this to be a concern.
Auto-x, you might be spinning one tire a lot, but if you keep it under control you won't have an issue. Want a leg up? If your class allows it, have an MFactory limited slip diff installed. This completely eliminates the weak spot.
The clutch? Great. Stock ones might slip but there are plenty out there that are up to the task.
Might want to look into a better fluid for the transmission. 80w90 or "Syncromesh" are quite popular, a major upgrade ove the factory installed ATF.
There are a few people who do legit auto-x, but they don't post here much. My friend Charlie in Arizona swapped an XR2 drivetrain into his festiva and races it pretty hardcore, and no transmission issues. Yes it's a lighter car and that can make a bit of a difference but he's got a LOT of track miles down.
My $.02
-Rocketman