Dogg,
Your book over simplifies it.
There was a range of G-10 performance depending on the engine installed. Some had the same engine as the Bf109K-4 and so had very like performance.
Pyro took that and originally decided to do a Bf109G-10 so that the top end Bf109 would have the option for either a 20mm or 30mm nose gun, rather than just the 30mm option as on the K-4. So basically he did a flight model for a Bf109K-4, gave it the gun option for the 20mm nose gun and called it a top end G-10. It was to give the players more options basically.
If you test the Bf109G-10 in AH right now you will find that it tops out at 451mph at 22,000ft.