The ultimates require a bit of aileron differential to straighten out the rolls (i.e. the ailerons need to go a little more up than down.) Without it, they do a slight barrell roll, which is most noticeable when rolling on vertical up-lines.
If you use rudder in your rolls, the second half of the roll will slow as you are using rudder opposite the direction of your aileron input. The top wing gives the same effect as having dihedral, so this gives the rudder power in the roll axis. This is fixed by programing rudder to aileron mixing. You'll mix it so that when you add rudder, the ailerons automatically counter with roll opposite the direction of the rudder input. You know it's right when your knife edges don't try to roll out on you. The rudder to aileron mix will reduce the coupling effect and help to even out your roll speed problem.
Biplanes take a lot of time to set-up for acro, but are well worth it. Once you have the mixing nailed, the planes are awesome.