As far as claims go, it's hard to estimate for Anzio because unlike at say, Sardinia, both Spitfires and P-40s were operating over the same battlefield simultaneously. But Stg1207 is making assumptions - you have several days where Spitfire pilots made 3 or 4 claims, and P-40 pilots made 10 or 15 claims, actual German losses were 5 or 6 aircraft and he assumes all of the P-40 claims were 100% imaginary and all of the Spitfire claims were 100% accurate. I think that's unlikely.
But what we certainly don't see are large numbers of P-40s getting slaughtered in action against Fw 190s or late model 109s at the time of Anzio, or against MC 205 or 202 fighters. I know of one incident in Feb 1943 in which the 33rd FG got massacred by some Fw 190s of JG 2, but that was their first encounter with the 190
I am not assuming anything, just repeating what is written MAW IV, and neither were the P-40 claims 100% imaginary nor the Spitfire claims 100% accurate; but you know that, we've both read the same book.
From memory, there don't seem to be a lot of encounters between P-40 units (USAAF and RAF) and JG2 in the MTO.