Weight is definitely easiest to fix, especially if you stick to shorter, sub 90 minute training sessions. I, and many that I've talked too, find that long, hard rides over a few hours tend to leave you wanting to inhale the contents of the fridge after a ride and this makes weight loss somewhat harder/slower.
If you want to equate weight to power - 1lb gained means you need 1 more watt on a 7.5% hill. If you can stand to lose 20 lbs then that's many watts for 'free.' The steeper the hill gets, the more the weight counts. Jamie - where do you typically find yourself losing ground? On the hills, flat, downhills?