I can remember a similar 109 thread a couple months ago that discussed the performance of 109's relative to each other. I would search it in the database but I have class in 10 minutes.
If my memory serves me right, I believe it was said that the G6 had an improved radio, better/more armor and maybe some other stuff in addition to the upgraded cowl machine guns and better engine(I think it had a better engine, i could be wrong). It was seen as an improvement because the improved radio and armor are things that in real life would have been VERY important to a pilot and very worth having, even if it meant it hurt your aircraft's overall performance a little in the long run.
I'm not sure whether or not HTC modeled the different armors in the G2 and G6 but that is likely due to the performance decrease of the G6 compared to the G2 due to the added weight, which the engine could not over come. Thus, hurting the G6's climb and top speed @ sea level performance, as well as turning ability.
I apologize if I have misspoke here, and I am not an expert. If I'm wrong someone please correct me. Just trying to recall from memory. I will look up the previous thread in question if I can. Correct knowledge is the main objective here.