I don't know about the GR16 but the Kraken already shoots plenty hot. The rule of thumb is every .05g above .20g causes a 30fps drop. Since the Kraken already shoots at about 370fps, even with .25's you're going to be shooting 340fps. That's still very respectable. I have a Kraken as a backup/loaner and I run .25's, I don't plan to make it shoot any hotter.
Most stock guns, especially the higher end guns, only shoot 280-320fps with .20's out of the box so you've still got miles over them. The lesser fps with .25's is deceptive anyway. Yeah it does result is slightly less range but not as much of a drop as you'd expect and it's much more accurate inside of that range. So basically you're trading a slightly shorter maximum range for a greater effective range.