For a stock class where amp draw is low, there is not much of a difference. The 17.5 TC Spec class @ IIC last year was won with 24C packs. In fact in stock, some feel lower C packs are better because they hold heat better, which makes the pack run better. For mod, I do feel that bigger C rated packs are more consistent throughout the run (not necessarily any faster on a hotlap, though).

The extra punch associated with a given pack is due to internal resistance, not C rating (although generally more C's = lower IR, but now always). This is why a "crappy" brand 50C may feel slower than a "good brand" 30C, because the cells of the good brand have lower IR.