I'm not at level 200 yet with any character (been taking a break to catch up on other stuff), but I kinda like the idea. Would give some incentive to those who want to keep using their capped characters, and the only benefit is a boost to RDR, so the cap boost won't really break the game over its knee (unless you think a +25% total bonus to RDR is broken even when you have a 0% bonus to your DAR). Besides, by level 200, characters are already powerful enough (and can cap/boost enough stats) to confidently fight Dark Falz and Olga Flow (I know there's the Ep4 boss, but the two I mentioned are, imo, more notable), so giving even MORE stats would be pretty redundant (to the point where bosses wouldn't even be challenging at all by 250, especially considering we don't have Ultima's increased Ultimate stats (thank god for that, though), nor a difficulty above Ultimate).
And I also have to say no to the prestige suggestion made earlier. Not that it's a bad idea, but it only really works when there's an actual reason to use it (ex. if I'm right, in Phantasy Star Portable 2 Infinity, you can reset your character's level to 1 (I think after reaching cap/a certain level), and in exchange you gain stat bonuses that make the additional difficulty mode easier/possible to clear (which can definitely help, considering the Phantasy Star Universe series has no way to increase stats outside of units, equipment, and leveling up characters/types afaik)). Not many people like the idea of resetting their progress (even by 20 or so levels) if there's no real benefit to it (true, Eden gives special equipment for prestige, but stat bonuses alone aren't enough if there isn't something more challenging to make those bonuses actually WORTH having (especially when said stat bonuses on the prestige equipment likely can't go over natural class stat caps. Correct me if I'm wrong on that, as I haven't played on Eden)), and Matt already decided against stat bonuses, so there's not much left that can be done with a prestige system to make it worth implementing over an increased level cap.