Well
It's not like Mk. IXc LF weren't in service over the mainland in 1944
So that's what you've got isn't it?
From a historical authenticity perspective I don't think there's any problem at all here.
The problem obviously is that you want to be competitive with the K-4 in DCS
Is the Mk. IXc LF (with 18lbs boost) not competitive with the K-4? I think it might be in some ways, you're not going to be eaten alive every time you fly just because it's a little bit slower than it would be with 25lbs boost