Not so much an obstacle as an anecdote.
WARNING: This story contains no math.
Several years ago I was training for a stage race as the first big peak of my season. I had a new team, with good sponsors and we were bringing a very good rider from Japan for this event. I knew the race had back-to-back mountain stages that would be crucial. I really wanted to be able to support him but I also knew that that type of climbing had never been my strength.
I started doing mad mileage and 3 day blocks of 5-6 hours of climbing each day with crazy elevation gains. After 8 weeks or so I really started to climb like I never had before. I started dropping weight (but was eating enough to train) and started getting even faster.
The guys I was training with at the time, some of whom normally climbed much better than me, became toys on the mountain. I was FLYING! I did a warm up race in Japan with tons of climbing and almost won KOM (another story)! Come race day I was down 15 lbs and couldn't wait for the climbs.
Aside from the 2 big mountain days there were a few circuit races, another point to point and 2 crits. Our GC guy won a stage and had a shot at the podium. We had to defend his position in the last 2 days and find him a few more seconds.
The second last stage was a crit. This had always been my strength but when it came time for me to go to work I just didn't have the jam. I had sacrificed a lot of pure wattage by losing too much weight (muscle mass) and, although still pretty strong, couldn't contribute the way I should have been able to.
In the end our guy slipped down a few spots on GC (another story) and I learned a lesson. Like Popeye says, "I am what I am and that's all what I am." Now I focus on lean and fit and don't obsess about my overall weight. (5'11" and a smidge and 172-175 lbs).
End of story.