Well when you look the class 70 only has 490 more bhp. But only has a topspeed of 75mph compared to 125mph of a class 67.
Surely, it's more about the way the power is converted to tractive effort that indicates how good a loco is for heavy freight. The Class 67 is a Bo-Bo because it's designed for high speed passenger / light load work. I seem to remember that they were originally intended for mail traffic, which is more about volume than weight.
The Class 66 has less engine power than the 67 (or a Class 91 for that matter), but it has a clever traction control system, which means the power is used efficiently, minimising wheel slip by applying the power gradually, no matter how heavy-handed the driver is (no disrespect to those drivers amongst you). This, together with lower ratio gearing mean that the 66 is suited to heavy haul, within the constraints of the UK loading gauge.
That said, freight operators would like a loco that has more grunt than a 66, which is what GE are trying to achieve.
CSRE is also offering a freight loco called Potenta, which will deliver 4000 hp, using two smaller engines rather than one large one, hopefully enabling operators to haul longer trains without double heading. This will have the advantage of saving fuel when idling or hauling empties, by dropping off one engine when more than 2000 hp is not required. GPS monitoring, together with line of route data will ensure that if a steep gradient is ahead, the second engine is automatically started to provide extra power when required.