Something about the K4 speed at high altitude
 

Looking at the bf109K4 il2 compare graphs, there is a bigger difference at 100 and 110% compared to the K4-C3, the G10 and the G14.
The other mw50 powered bf109s have a 25km/h speed difference between 100 and 110%, while the K4 has a 45km/h speed difference between 100 and 110%.
This speed difference of 20km/h~ is very obvious, comparing to the other mw50 powered bf109s and it could use a fix.
This would mean less overheating and better performance.

I've noticed that too. Reduce your throttle. ;)

It is nice to have that extra "go" from 100% to 110% though, so I only use it when I'm in a clinch. Or if I want to catch up on a P-51.

The topspeed of the ingame K4 is 20km/h faster than the real one.
Thats why this difference is there.