The G6 had a reinforced structure landing gear, extra armour and reinforced armament. This brought the weight up by 300kg, and the engine power remained the same. The aerodynamics were negative affected too. As a consequence the performance of the G6 was far below than the G2.
Regards
Thats not quite true
Apart from a different radio set, 13mm MG131 machine guns instead of the 7,92mm MG17s which were the reason for the cowling buldges, a non-retractable tailwheel, bigger wheels and thus the necessity for the kidney shaped wing bulges the G2 and G6 were identical.
Weight difference is also far smaller than the 300kg you claim
Historical weights for Bf 109 G series obtained from primary sources (GLC charts and Hanbuchs) only:
Weights for Bf 109 G-2 Fighter with DB 605 A-1.
Empty Weight: 2253 kg
Equipped weight: 2580 kg
Weapons:
Armor: 78 kg
Crew : 100 kg
Fuel weight : 298 kg
Lubricants weight: 33 kg
Munitions weight : 80 kg (30kg + 50kg)
============================
Take off weight: 3100 kg
Weights for Bf 109G-6 Fighter with DB 605 A-1 (no MW50 equipment) and recon cameras installed. 20mm cannon.
Empty Weight: 2268 kg
Equipped weight : 2679 kg
Weapons : 135 kg
Armor : 78 kg
Crew : 100 kg
Fuel : 295 kg
Oil : 33 kg
Bombs: -
Ammunition : 88 kg
MW 50 : -
---------------------------
Take off weight : 3196 kg
Weights for Bf 109G-6/R2 Recon with DB605AM and MW50 injection and recon cameras installed. 20mm cannon.
Empty Weight: 2268 kg
Equipped weight : 2740 kg
Weapons : 135 kg
Armor : 46 kg
Crew : 100 kg
Fuel : 296 kg
Oil : 33 kg
Bombs: -
Ammunition : 88 kg
MW 50 : 63 kg
---------------------------
Take off weight : 3320 kg
Weights for Bf 109G-6/R5 Recon with DB605A (no MW50 equipment) and recon cameras installed. 20mm cannon.
Empty Weight: 2266 kg
Equipped weight : 2701 kg
Weapons : 135 kg
Armor : 78 kg
Crew : 100 kg
Fuel : 296 kg
Oil : 33 kg
Bombs: -
Ammunition : 88 kg
MW 50 : -
---------------------------
Take off weight : 3218 kg
Weights for Bf 109G-6/AS Fighter with DB605ASB (no MW50 equipment). 20mm cannon.
Empty Weight: 2293 kg
Equipped weight : 2704 kg
Weapons : 135 kg
Armor : 78 kg
Crew : 100 kg
Fuel : 295 kg
Oil : 33 kg
Bombs: -
Ammunition : 88 kg
MW 50 : -
---------------------------
Take off weight : 3221 kg
Weights for Bf 109G-14/ASM Fighter with DB605ASM (MW50 equipment). 20mm cannon.
Empty Weight: 2284 kg
Equipped weight : 2679 kg
Weapons : 135 kg
Armor : 46 kg
Crew : 100 kg
Fuel : 295 kg
Oil : 46 kg
Bombs: -
Ammunition : 88 kg
MW 50 : 63 kg
---------------------------
Take off weight : 3272 kg
Weights for Bf 109G-14/U4 Fighter with DB605AM (MW50 equipment). /U4 = 30mm MK 108 cannon.
Empty Weight: 2294 kg
Equipped weight : 2724 kg
Weapons : 170 kg
Armor : 46 kg
Crew : 100 kg
Fuel : 295 kg
Oil : 46 kg
Bombs: -
Ammunition : 89 kg
MW 50 : 63 kg
---------------------------
Take off weight : 3318 kg
Weights for Bf 109G-10 Fighter with DB605DB (MW50 equipment). 20mm cannon
Empty Weight: 2318 kg
Equipped weight: 2704 kg
Weapons : 135
Armor : 46 kg
Crew weight: 100 kg
Fuel weight : 296 kg
Lubricants weight: 46 kg
Ammunitions weight : 88 kg
Bombs: -
MW50 : 63 kg
============================
Take off weight: 3297 kg
Hereby you see the Bf109G6 and G2 differed only by a megre 96kg on takeoff which hardly has an influence on top speed itself but on acceleration and even then the difference is insignificant. The only thing that really makes a difference speed-wise between the two are the small aerodynamical changes such as:
- the non-retractable tailwheel
- the cowling bulges
- the wing bulges
All in all this accounted for something between 10-15km/h of speed loss. Now given that the Bf109G2 is stated to attain a max speed of somewhere in the region of 656 - 666km/h (the latter confirmed by Soviet test performed at 1.3ata) we arrive at a top speed of 640km/h for the G6 in case of a worst case estimation (656km/h for the G2 that is).
The 625km/h topspeed that are so often claimed for the G6 stem from an RAF flight test in which the respective G6 was carrying the Rüstsatz 6 (i.e. the 20mm MG151/20 gondolas) which added up both to the aircrafts weight and its inflight drag. Coming this way its understandable that a majority of LW pilots were less than thrilled about having gondolas mounted to their 109s as they affected overall performance quite considerably (speed, climb, turn, rollrate, etc.)
Now these 640km/h figures are valid for an early 1943 built Bf109G6. Some while later in late 1943 - early 1944 the DB605A was finally cleared for its initially planned 1.42ata boost pressure providing a full 1475PS at takeoff. The extra 150 PS from this increased boost setting increased top speed of the G6 further to roughly 655km/h, so back to the same performance that the G2 had in mid 1942.