Sluggishness
Have you checked the setup of the props, and done a check-calibration of the RPM indicated in the cockpit against an optical tacho? I've seen this on a few singles where performance was well down, and it turned out that the prop pitch (or occasionally profile) was wrong, but this was being masked by a misreading cockpit tacho.
Rolling
I wouldn't be surprised if a small amount of offset in the fin did that in roll, it's a big fin! But, could you remind us how the yaw and (if it's got one) roll trimmers mechanise on the Dak?
G