Only reason i can think that people get a benifit from the alloy rad is that it's new and clean.

Also, the XR2 bay is pretty cramped, and im sure that will make a difference. I'd certainly be very interested if you do change the rad. I still think a lot of it is where the temp is measured. Example, ive had thermcouples on the head and it never goes near 100, but the water as it exits the head past the last exhaust valve etc can get hotter. Never done any track work in mine, so it's never been properly stressed.