My Testing at 22400ft, 29.92inHg, 20 degrees Celsius, 50% internal fuel, 54.9inHg manifold pressure and 2700rpm yielded a maximum level flight speed of 240kts indicated. Calibrating that using the manual is 246kts, then plugging that into an online calculator for TAS i got 385kts. then converting to mph i got 443mph which is 18mph faster than the real plane and is in fact closer in speed to the -4 variant with a maximum level flight speed of 446mph. Attached are screenshots from my run (ignore the slight rate of climb, i bumped the stick while pausing and it was less than half a second of climb) and of the manual and calculator used (the conversion from Knots to mph i did on my phone).