THRASH99,
12,000lbs vs 8,000lbs on the same power is not a slight difference. The Spitfire Mk XIV raises it up to about 8,500lbs, but on significantly more power.
Why would you expect the 12,000lb fighter to climb, accelerate or turn as well? Don't get your history from "The History Channel" if you care about accuracy. The Spitfire was a significantly better fighter in a duel than the P-51 was, but that doesn't help when the fighting is out of range of the Spitfire. There were many American Spitfire pilots who much preferred the Spitfire over the Mustang and about the best you get from them is that at least the P-51 could get them to the fight.