Well according to Davide Cassani on the Italian RAI cycling coverage, a top grand tour athlete (obviously who has not worn himself completely out and is going on reserves) can save circa 1min. 30 sec. on a ten mile climb at one kilo less.
Don't know what scientific criteria upon which Cassani's claim is based, though I'm sure it has some basis in reality.
From my experiences, when you train properly and consitently, eat sufficently at the table but not more, and be disciplined and focused in both areas, your body will arrive at it's ideal power-weight ratio naturally. Especially when you have confidence in what you're program is building and the patience to avoid exaggerations that you think will get you more when in fact they break you down.
When I was going fastest uphill, I weighed between 57-58 kilos and, if I put on just two, three kilo's, I could not keep the pace high for as long.
It seems to me that was the real issue. It wasn't that at 61 kilos I couldn't climb on the same gradient just as fast as at 57, just not on the same gradiant for as long. That was the difference. When 57 was the result of a natural process. The Italians say: Nel ciclismo non si puo inventare niente. Piano, piano, si va lontano ("In cycling one doesn't improvise anything. Slowly, slowly, you'll go far). Where "slowly, slowly" more properly speaking means "patience, patience."
What gets you down to the ideal weight? A combination of miles and intensity (racing) over a sustained period, say from Feb. to June. After that you're flyin as they say. In a relative sense, of course. Sombody always goes faster!