My guess is that there is some point of diminishing returns. As an extreme example (and all too common): a runner starts to lose weight and gets faster and as the weight comes off the performance improves. The runner thinks "Wow, if I just keep losing weight I am going to win ______".
However, at some point the runner is going to reduce energy intake and workouts will start to suffer. The runner may get sick or just not have the energy to train or perform well and times start to creep up again.
My suggestion to folks is to train well and eat well enough to support the training and the speed will take care of itself. I also think that people have an ideal training weight that they can maintain for much of the year and then when in the key competition period drop a FEW pounds and be sharp. Then the weight comes back on. (Not in a Lolo Jones way, but that is a pretty unique case).