Who the hell even flys anything that high outside a MoM mission!? (Not an invitation to tell me you do, guys) IMHO, that statistic is irrelevant. The 109's loose performance at that alt due to their superchargers, not their props.
Irrelevant? Dude, the 47 is a total beast up there. NOTHING comes close to it. In FSO's, scenarios, and the like (not to mention Combat Tour when it was still being worked on), that's it's playground.
The 109's props weren't called into question. The 47 D-11 doesn't have one, nor does it have a MP boost available like it should, simply because everything is modeled as "factory stock".
Adding these two things to the D-11 will make it accelerate faster, climb better overall (making it even more dominant up high), and give it even more of a speed advantage.
I don't think the designers at Republic had Aces High gameplay in mind when they were designing their machine to kill Germans at high altitude