I have found if the roll rate is speeding up, you're letting the nose drop too much. So keep it up, through feeding in rudder and elevator and it will stay nice and slow and look better too. As far as altitude is concerned, remember in a normal harrier one of the best ways to descend is to pull the nose up without adding power. Same thing in a rolling harrier. Get the nose up and let it slow down and it will begin to descend. You can then impede this descent by adding power. I find I'm working the throttle all the time in this move. Hope it helps!
Dave