Here in Norway, at least three Impact owners have had the same rolling problems as described by others in this thread.
I've investigated this in numerous flights and my preliminar conclusion is that the need for negative differential is a function of how much aileron that is actually used. For example, if i set up my Impact to roll perfect in vertical half- and quarter rolls (relative fast point rolls), the programmed amount of differential makes slower, horizontal rolls very difficult.

I beleive this has to do with nonlinear effects of the sharp lip that opens up on the underside of the aileron.
If this is the case, making the programmed differential a function of stick input would theoretically help, but my radio (Graupner MC24, and I believe other radios as well) has no convenient way to do this even it is possible.

I have now sealed the gap with tape and hope this will reduce the problem. In addition to this I have moved the servos from within the wing to the outside the enable perfect linkage geometry.
However, winter caught me before the mods could be tested.
The first tests will now be in april when the snow is gone.

Oistein