'Realist bomber speeds' is governed by engine endurance...
Russian mass produced generic aircraft seemsed to have had the most issues with overheating...
Guessing that German Engineering had precision and quality, so the least...
USA and then Britain probably between those two...
Our plane's WEP kinda covers this stuff...
Really though, the end result of keeping the pedal to the metal would increase the 'chance' of blowing an engine, hence something somewhere fails. Yeah though, temperature would have to do with it. Hunting down this type of info wouldn't be easy, since 'how long a plane could maintain its speed isn't usually recorded.
I agree that it can be frustrating if strugling to catch up to bombers when they seem faster and/or outclimbing a fighter.
See, wartime, bombers flew together going in one direction, in game though, independent 3-sets of bombers just turn a little and stuff. So, fighters trying to get ahead for the ideal head on attack isn't easy.