Well everybody knows that just painting black crosses and swastikas on an aircraft adds 20-30mph in speed and hundreds of feet per minute in climb
Not mention the guns shoot further and more accurately
However the US always planned to replace the P-39 and P-40 with P-38s and P-47s as soon as increased production of the P-38 and P-47 allowed. The P-51 had no bearing on production/deployment plans made in 1941 and early 1942 and please remember that these plans were often being made a year or more in advance. Sometimes two years and sometimes plans crashed and burned, like Curtiss production of P-47s.
US ordering of Allison P-51s was by the hundreds while orders for the P-39, P-40, P-38 and P-47 were by the thousands. It is not until the summer of 1942 that investigations start on using the Merlin in the P-51 and orders for the P-51 go to thousands per order in the fall of 1942 and spring of 1943.
I would not put a lot of stock into some the Russian stories either. In some cases there are just too few reported incidents to make a valid judgment. Much like the Buffaloes lousy reputation being based on about a dozen planes at Midway on one mission with green pilots.
I would note that some of the combats reported using Tomahawks in Russia are from Jan/Feb of 1942 or the Germans first winter in Russia and one might wonder just how well the German planes were maintained of the condition of the German pilots.