Lower pot value = lower/flatter resonant peak = less bite, smoother tone
I think "aggression" is somewhat subjective, though... Do more low, growly mids make a pickup sound more aggressive to you? Or does a punchy, biting upper midrange focus sound more aggressive?
The former might sound more aggressive in a single guitar arrangement with fewer instruments competing for that space, while the latter might sound more aggressive in a multi-guitar arrangement or with busier instrumentation.
However, in general, most people are probably going to find a JB w/ 300K "tamer" than a JB w/ 500K+