Bold statement. Stepping up to the G-6, I see a plane that is faster at most altitudes under both mil and wep. (16-22k exceptions) It also climbs significantly faster under 11k under both wep and mil, and has only a minor disadvantage way up in the clouds. I think you are also underestimating the difference in hitting power and ballistics between the 7.6 and 12.7 mm guns. I am fairly certain that if the G series was inferior, they would not have produced quite so many of them. I'm certainly no 109 expert, though.
Our g6 is an attempt to find the middle ground on the 'g6'. The g6 was a smörgåsbord of field mods, etc... There were 6's with different canopies, different tails, different hub guns, different engine mods, different clearance for max manifold pressure, etc.. On top of that, ground crews were always moding them, tuning the engines better, etc... on and on it goes...
It was so bad the the G14 was really an attempt by the Germans to standardize the 'g6' but even some g14s had different engines etc... It is more like the k4 was more consistent in its specs.
Personally, if your going to go with a 'g' model, I'd go with the g2 or the g14. G2 in my opinion was the 'best' 109. G2 is much better than the in game g6, and the g14 has the 30 mm which changes the fight entirely.