So yeah - I put the wrong radiator in my brother's Pontiac Grand Prix. He has a 2004 GTP Comp G with the supercharged L32, and I put the radiator for a naturally aspirated L26 engine in it. The end tanks are the same, but the L26 radiator's core is 5/8" thick and the L32 core is a full 1" thick.

Hopefully this will resolve the "slow overheating" issues. The car would get up to 220 degrees (the temperature that the fans kick in) even when you weren't pushing it. I got it to 220 with the thermostat removed running it in Neutral at ~2000 RPM without it throwing a P0128 (thermostat problem) code, so it seems pretty clear that the thinner radiator isn't cooling well enough.

I'm planning on doing a thorough flush and swapping to Peak Global Lifetime (yellow) coolant instead of Dex Cool. We did the lower intake manifold and supercharger gaskets about a month ago.

I feel really bad about leaving my brother with a car he's afraid to drive. I hope I can make it up to him somehow.

Insert obligatory comment about getting someone with the handle "NaturallyAspirated" to work on your supercharged car.