Attempts to streamline production rarely, if ever, bring about the speed increase like it was the case with 109K vs. late Gustavs.
Granted, Luftwaffe should've insist on the Gustav to retain the retractable tailwheel (already present on the G-2). The addition of wheel well covers was no rocket science in 1942, either. But what was really lacking was a major increase of power, even the fully rated DB 605A was just a small an improvement over the DB 601E, by what time the Allies are neck deep in introduction of far more potent engines (Sabre, Merlin 60 series, Griffon, R-2800 with turbo or with a 2-stage S/C).
The firepower will need some thinkering, the G-2 was well behind he curve there as-is.