The cooling on the oem fmic is fine, the issues are the aircon rad fins becoming bent and limiting are flow to the fmic core which causes elevated air temps.
The oem fmic has a pressure drop of around 3-4psi from hot side turbo outlet to the cold side which costs around 20-30bhp in flow efficiency.
I have found the S3 alloy fmic setup work best on the GTi k03 setup for stage 2, I recently tuned a Leon Tfsi K03 to 300lbft 293bhp just using a turbo back full exhaust, S3 alloy fmic, a Forge twin take and 5 mm actuator adjustment. There is NO need to upgrade the HFFP as I have found the OEM unit will work happy at 280-295bhp at 120bhp with the k03.
Nick