Well, considering that one of Gren's weakness is his general lack of controlled mobility, I'd say pretty effective. While he's certainly not sluggish (As I keep saying, get into melee range and you can see just how fast he can swing The God Eater) he does lack the ability to keep up with most speedsters. With the rocket boosters that ought to give him a burst of speed to close the distance, as well as make his God Eater assisted flight more manageable. And of course there are other uses of the rocket boosters too: He can direct and adjust their angles freely, allowing them to be turned into anti-personal flame throwers by aiming the exhaust towards his opponent. If he can Friction Stick them over the exhaust, and he'll BBQ them.
That being said, these rocket boosters alone won't be enough to grant him full on flight, considering his weight. Maybe slow his decent or boost his jumps. But less so if they have to resist the force of shooting The God Eater, since the recoil alone turns him into a kinetic missile if he's not firmly rooted to something solid. Though I would also like him to eventually upgrade to what will essentially become a jetpack, and I'm willing to pay additional upgrades if necessary.
