I'm jumping into this discussion a bit late, but I sorta disagree with your statement that there is no natural air flow through the radiator. I've always understood the that Mini's left front wheel arch generates a low pressure area at speed, so the air entering through the front grill of the Mini is naturally drawn through the side radiator into that wheel arch cavity. The fan, of course as with most cars, is only necessary at lower speeds, or when stopped, to help maintain the air flow through the radiator. Also, the reason for the switch to the front mounted radiator in the Mini's last few years of production was not to improve cooling (although that may have been the result), but instead it was done to comply with increasingly stringent EU drive-by noise standards. The Mini's exemption ran out, and Rover/BMW were forced to finally mount the radiator behind the grille to achieve a quieter drive-by noise level.
I may have a small amount of real world experience to offer here too.... While fussing with Buzz over the heating problems I was having, I removed the engine driven fan to see a) would it still cool b) would it be any quieter and c) would the electric fan be enough to cool it by itself. By this point my car was no longer running hot, but now the opposite - despite having a 180° thermostat - I had changed the water pump pulley to a smaller one and that seemed to be the difference. Answers to the questions are : a) no, it doesn't seem to flow enough without the fan to cool the engine - it started getting hot on a 50° day. b) yes, it was quieter, but not enough to notice till I put the fan back on again c) yes, the electric seemed to bring it down again eventually and keep it there. So, I re-installed my engine driven fan and I'm back to running 165° again.....at least when it's below 50° I don't know about the radiator in the front thing, but I'll bet it was a combination of the noise and needing to keep the temps in a narrow range for both emissions and fuel economy on the fuel injected engine. One things for sure, the radiator in the front does cool the engine more efficiently.