A couple of points gents:
The vast majority of 100+ victory Luftwaffe aces gained the largest percentage of their kills against the Soviets. Considering the state of Soviet aviation in their first two years of the war with Germany, that should not be a surprise. Indeed, the Finns abused them too, and usually flying lesser aircraft than the Luftwaffe.
As for Japanese and Soviet individual scores, historians generally believe that if you take their kill totals and divide by 3, you come closer to the actual totals. Both the Japanese and Soviets handed out "confirmed" kills like candy on halloween. Japan was especially generous in this regard, honoring almost every claim. Typically they over-claimed by a factor of 4, the highest of any major combatant in the war.
As to the Soviets, if you factor all claims of Luftwaffe aircraft and compare that to the total number fielded by the Germans, the Soviets would have killed the entire Luftwaffe (beginning in September of 1939) twice over. This doesn't mean that the Germans, Brits and Americans didn't over-claim, they surely did. However, many claims were disallowed by far stricter rules of confirmation than were employed by the Japanese and Soviets.
Many of the British pilots shot down in 1940 were recovered and were back in the air almost immediately. Likewise for German pilots defending against Allied bombing attacks. I believe that Hartmann was forced down 14 times during the course of the war in the east. Unfortunately for the Luftwaffe, there was no other option but to keep pilots in combat until they were dead or injured too badly to fly. That certainly contributed to their high kill scores. But, it should be stated that their level of skill was extraordinary by any standard.
My regards,
Widewing