The problem is that the maximum power rating is irrelevant for hauling a train up a hill or away from a standing start, for that you need a high tractive effort.
The power being put out by any motor at any time is basically the torque multiplied by the speed - this applies whether you're talking about a traction motor (or group of traction motors) on a rail locomotive, the prime mover in a diesel-electric locomotive, the motor in your car or even the power applied by a person pedalling a bike.
Gear ratios can be used to adjust how the motor's power is then applied to the motion of the machine, as either a heavy duty tractive effort with a low top speed or a lighter tractive effort capable of higher speeds. If you have multiple gear ratios (like in a car, on a bike with gear shifters, or on a diesel loco with a mechanical transmission) then you can change gears to have both good effort at low speed on a hill and a lower effort keeping it running faster on the flat.
Diesel-electric and electric locomotives get only one gear ratio as they don't have a transmission with multiple ratios available at the flick of a shifter. It's easier on modern locos with better electronics, but for older locos changing the gearing is a major workshop job.
Applying this to the 86/6 issue, it is my understanding that the gearing on the 86/6 sub-class is set up for a higher top speed which gives them a lower continuous tractive effort than is needed to run a full-length Freightliner train. Freightliner had 86501 fitted with an experimental shorter gearing which gives it that higher tractive effort at the expense of top speed. While the 86501 conversion seems to have been effective, that it hasn't been repeated would suggest that it was quite an expensive process (i.e. more expensive than running the 86/6 locos in pairs) or that there were other reasons for preferring to run pairs (redundancy during failures?).
For a more obvious example, a pair of HST power cars puts more power on the rails than a Class 60 medium freight locomotive (there are no heavy freight locomotives in Britain) but you don't see HST power cars hauling even the relatively short freight trains which run in Britain. Why? It's because they have an extremely tall gear ratio for a huge top speed but such a weak tractive effort that they pull out of stations slowly and are only capable of hauling a short rake of lightweight passenger coaches. Even a SAR 830 Class light freight loco here in Australia has a higher continuous tractive effort than the maximum tractive effort of a pair of HST power cars - despite having less than half the power of only one power car.
