If you a fill an impetus from nothing with charged amethyst it caps at 200,000 media and then stops accepting media via hopper. However, after casting with the impetus it can be filled past 200k, and the media value can overflow. I suspect it has to do with non-whole number media values, but that's just a guess.

Further investigation reveals that impetus's will only lock if they hit 200k exactly, so if they're on 199,996 and you hopper in a shard they will pass 200k and continue to fill and eventually overflow.