If you find your themo fan is not up to the task..here are some approximate calcs....
By putting the fan behind the radiator and
not removing the original shroud, you reduce the potential flow by about 25-30%. Which would be fairly critical at highway speeds. Hence why you still need the mechanical fan on the highway!
On top of that if you have left large gaps around the fan, you have reduced the available area by 25-30% at idle when the fan is working.
This is greatly compounded if your fan is set back from the radiator with
no added shroud, because the fan is going to suck air from the places of least resistence......all around the fan and
not through the radiator, even if it's working at a zillion revs it's probably doing very little to cool your hot engine at idle. Hence why you still need the mechanical fan at idle!
It's a lot of work in a mini to have an electric fan work as it should because the radiator is held in by the fan shroud, so another method of support needs to be made.
Edit> A bit of a balls up in the calc

Thought it was a bit exaggerated! It should read about 10-15%
Still you can see how electric fans would not necessarily solve the problem.
Another problem at highway speeds is the mechanical fan is thrashing about unable to cope with the
high speed flow, so becoming a hinderence.