Yeah rads are inducing drag. The default's CoD FM is correct.
Of course if you are flying with some amount of side slip or at a low pitch you won't notice any difference
For the correct settings, don't forget that at low alt it would be more difficult to cool oil/glycol fluids in most situation -> the radiator opening would hence have to be wider (a strong point for cruising at alt
)
@ Kein : you can reach easily 480/490 level with the E.