D2 was the testbed loco for the Cl 45s - D2 for a period and the Cl 45s were fitted with an intercooler between the turbocharger and the cylinders. (turbocharged air is very 'ot, caused by the compression)
Because the intercooler cooled the intake air (funnily enough!) it meant there was a greater mass of air, ie it contained more oxygen, in the intake air of the stroke of the intake cycle, so you could get more fuel burned, so you could get more ooomph (ie power) out of the engine per power stroke.
This is why the Cl 45s were rated at 2,500 HP and the Class 44s at 2,300 hp (IIRC).
I would strongly suspect that when fitted with the intercooler the Derby engineers were monitoring the locomotives performance and engine characteristics of D2 very closely. 200 extra horsepower on top of 2,300 may not sound a big change, but it would have involved all sorts of extra problems - higher temperatures of exhaust gasses, greater forces on the con rods, crank shaft and bearings. And, potentially, you might have had extra problems on the electrical side too - though with the Crompton electrics, I think this again more or less coped.
The Cl 45 just about worked, but when they tried to push the engine up another 200 hp for the Cl 47, it all came down with a big bang, and they were derated to 2,580 hp, because the extra 200 hp was just too much for it - the straw that broke the Sulzer camel's back, as it were. I don't know what the critical elements were - someone in here will probaby chirp in with the answer(s).
We may have seen some of the same trains on the days that I was at Bletchley! I have nver understood why the EE4s were sent to the WCML where, as you say, trains were often super-heavy 14-15-16 carriages, and the more powerful Cl 45s went to the Midland, where the maximum train length was (I think) 11, but more usually 10.
I don't know if it's apocryphal or not but it may have been a case of management seeing 2,000 hp in the locomotive report and thinking: "Wow. With steam, we struggle to get 1,500 hp - this must be good," - but not realising that the 1,500 hp for steam was the drawbar horsepower rating, while the 2,000 hp for the EE4 was the ENGINE rating - you then had to factor in the losses in the generating-traction motor system, and the fact that you had to move 137 tonnes of EE4 locomotive before you got your drawbar hp.
All trainspotters, when they go to the pearly gates, will be called at a special time, when St Peter will be accompanited by Ian Allan, working as a consultant. If you have cheated on your cops, Merle, you could be spending a lifetime in spotting purgatory, ie in your case at Northampton shed just pror to closure, with three class 4Fs outside with numbers covered in grime (so you can't read them), a continual westerly wind allowing you to hear Stanier pacifics working expresses on the Weedon route, and only a lonely Class 08 shunter inside to spot, day after day, night after night... total boredom. You will suffer this until you satisfy said team that you have learned your lesson, and will not cheat on numbers again!
