Keep in mind that when the G6 was first in action in spring of '43, it's primary fighter opponents were much more in it's league then the typical opposition in AH. In Western Europe it would have squared off against Spitfires, a few Typhoons (almost exclusively used for ground attack missions), and P-47C's. In the Mediterranean it would have been a mix of Spits, early P-38's (less horsepower, non-boosted ailerons), and P-40's. On the Ostfront it's primary opponents would have been LaGG-3's, Yak-1's, Yak-7's, P-39's, P-40's, plus a few Hurricanes, Spits, Mig-3's. The very first La-5's were starting to be introduced and weren't too common yet.
In these environments (especially in the east) the G-6 was still a first rate fighter. The only problem is that a year later the G-6 was still the standard Bf109 in service and it was now facing P-51's, P-47D's, newer P-38's, Yak-9's and Yak-3's, La-5s and La-7s. It goes from being about as fast as it's best opposition to being 30+mph slower in many cases. It wasn't until late '44 that the G-10 and K-4 started to come into service, and by then the quality of the average Luftwaffe pilot and the odds they faced had reached a point where it didn't matter.
