With a large oil cooler in front of the radiator, I noticed that the water temperature exceeded 115 °C. At such temperatures, the ECU starts to pull timing to reduce engine power output. I resealed the radiator shroud, cleaned out debris, installed a 160 thermostat (71 °C) and lowered the fan settings with HP tuners, but this was still an issue.
The stock radiator had developed a hairline crack in the plastic end tank, which I discovered by luck when running the engine without the radiator and the fan shroud removed.
A DeWitts or Ron Davis radiator was my preferred solution, but after adding shipment costs and import taxes the total was above $1000. I decided to go for the less expensive Engineering Cooling Products (ECP) radiator for the C5.
The ECP radiator arrived without any visible damage, which is always a risk when having fragile parts transported long distance. I bought it with a consumable electrode, to replace the flimsy petcock supplied.
The radiator installed nicely and snug, except that I had to remove the inner lower tab for the condenser (see red circle).
To change the radiator, the fan shroud had to be removed. A typical plastic fantastic GM design, which I quickly decided NOT to reinstall. This also improves the airflow through the radiator core, as the airflow is not blocked by the shroud at speed. The dual fan setup (2x800 CFM) is replaced by a single 2500 CFM 16" Spal fan.
I've not been to a track day yet, but I can tell from regular street driving that the cooling efficiency is much higher. Not a surprise when you look at the massive difference in size.