last year i dropped my weight down to about 9st 8lbs in dec from 10st 4lbs in aug. i trained away and come race time i sucked. i realised that i had lost some power and paid the price in top speed. so i changed my diet, gained all the weight back and kept training. my results improved and i enjoyed my racing!
this year the focus is on training and quality nutrition, not losing a few pounds. with the style of courses we ride here in n.i, its about power first, then endurance.
i think if you need to shift a LOT of weight in fat, then do it, but if you are fairly slim, focus on hard training, not calories. those last few lbs arent the problem, imho. however, i can feel the fat slipping away as my jeans are getting loose, but the scales dont show much, and that, i'm pleased with.