If there is ample amount of food, do C. elegans worms know when to stop eating or do they store extra energy? Could they put this extra energy to use by moving faster or putting more eggs?
The answers are:
Yes, they know when to stop eating. See this paper.
This one is a little more complicated. They can use extra energy, but not to move faster. Movement doesn't take much energy, and until they're close to death from starvation, they continue to move quite well. As for eggs, they do indeed slow down the rate of embryo production when nutrition is inadequate. But the total number of eggs is limited by the supply of sperm, so there is an upper limit on how many they can produce, even in unlimited food. Probably the main benefit of stored nutrients is to allow them to survive longer if the food runs out. They are very, very good at dealing with a variable food supply -- much better than you, for instance -- and they do it partly by storing up extra energy. This paper and references therein describe some starvation responses. You can also look up L1 diapause, the dauer larva, and adult reproductive diapause.