In reply to The Ivanator: HI
I would say that in a sport governed by strength to weight ratio loosing weight (to be at a optimum) is paramount. HOWEVER as has been mentioned there is a lot more to it than that, personally I think that you have to be a good/established climber to start with.
My experience in the last 12 months may be of interest. I have been living in Bolivia, guiding one treks and high mountains got me ridiculously fit CV wise and down to 12 stone from about 13 (Im 6'1) and my climbing grade improved however I only had limited easy sport (up to 7a) and some bouldering to go on. (probably rock climbed about the same frequency as in the UK)
What did improve was my was my endurance, I had a couple of projects, scary E2/E3 with crap/no gear and was happy hanging around on them all day, just no pump, no heaving a hefty lump on big holds.
Then I had a stint back in the UK where I was working on a building site and eating like a monster I got ridiculously strong (think a one man fridge lifting machine) I never checked my weight till the end of the 3 months although I knew I was getting bigger, I was 14.5 stone and lot of it was muscle. Interesting I was in london and only had access to a bouldering wall twice a week.
My grade actually went up bouldering wise - Flashing Font 6b+ and working 6c/6c+. what I did struggle with is endurance. I would burn out quickly. I went to N wales for the last week I was in the UK as I was instructing and Logging for my MIA and I could still walk up HVS/E1, just didn't like small holds!
Im now on a 2 month drop back to 12ish stone and aim to be 11&1/2st by September end as I go to Yosemite. From previous experience this WILL make a difference particularly to endurance. Interestingly the strength that I gained has not left as I get lighter in a particularly rapid way (which is not recommended)
Rob
So I would say if you are an established climber