I'm not sure if the OP knows this but since it wasn't brought up yet, I will. The "C" rating by itself is irrelevant - it's the C rating multiplied by the pack's mAh rating that tells you the maximum discharge.
So assuming you're talking about a 15c 4000mAh pack vs a 20c 4000mAh pack, yes the C rating is indicative of the performance differences. However a 15c 4000mAh pack has the same discharge ability as a 20c 3000mAh pack - 60 amps.
Just wanted to clarify that in case you didn't know - the C rating alone isn't what's important.