Probably second, as the more heat the IC transfers from charged to the environmental air, the
hotter the latter is getting, and the less it's able from suck heat from the radiator and coolant.
I'd go OE auxiliary heaters. They do work on the R and they certainly will do the same on a GTI.
The only downsides are cost + you'll need to gain the necessary space, but other solutions will
likely entail very similar challenges.
On a side note: Did anyone consider using the TDI fuel radiator for engine oil cooling duties? TDI
Golfs use or used (I remember the Mk5 TDI had) to have one below the passenger seat. It looked
like an amplier heatsink.
Btw, I remember Mishimoto making a radiator for the Mk.5 and 6.