Personally, I'd be fine keeping Phlegma's potency the same since it's not exactly a 100% increase by halving the Cooldown because you have to factor in the opportunity cost of less Dosis casts. Not to say that Dosis is stronger than Phlegma of course, but rather if you're caluclating the total possible potency per minute you can dish out, you have to take out a Dosis first to put in another cast of Phlegma, so it's moreso a potency gain of 270 potency on GCDs that replace a Dosis rather than a flat 600 boost.
But as far as potency bloat is concerned, something that's been talked about before as well is twofold attacks, or other examples, like Dream within a Dream. Given Phlegma's Animation, I think you could absolutely split the 600 potency into two hits: one during the initial animation, and another when the energy sphere implodes on itself.