Fiecos Dan
Dan, the core's have changed over the years. I've uses 70 quid copper rads, non OE and they have been superb. I haven't been on track, but with the very thick cossie cooler covering the rad, lots of very fast road work as well as traffic, the rad never struggles.
I don't know which brands are good and which aren't for alloy. Maybe because they are physically thicker they cool better? There are firms that make custom copper rads - maybe this would be the ultimate?