They needed the firepower. Not all fights were "dogfights" -- even if they were against other fighters. In fact, most weren't. You get a good angle on a target and you press the attack. In that case a couple hundred pounds of lethal killing weaponry would make the fight end in a heartbeat. It was only when you were caught in the bounce and at disadvantage that it would cost you dearly.
Not all improvements that keep you alive enhance your plane's performance. Self-sealing fuel tanks, for example. They add weight, reduce handling, reduce fuel tankage, but will save you from even minor damage. Better radio gear, for example, might add another 80 pounds to your loaded weight, but if you've got good combat communication and tactics/teamwork, it'll save your life and kill the enemy just as well as any extra guns will. The P-51 bubbletop, for example, added drag over the bird cage canopy. It took several mph off top speed, if I recall. It was life saving in keeping your SA open before and during combat engagements.
So yes, the later 109Gs didn't have as good an advantage against their closest competitors in the RAF, but they were still every bit as much of a threat overall.
