In his book "Edge of Darkness", Barry Thornton mentions a focusing problem with his SL66 during some tests (pgs 46-48).
He was using a homemade target with equally spaced vertical lines, focussing at the exact center.
His fix was that the interchangeable screen insert was not seated properly.
This test might be something to try. I am also having problems with the SL66. At first I thought it was my poor vision (I am using the "default finder" not the prism finder). Even with magnifier glasses, I still can't get the sharpness that my Mamiya 645 can achieve.
I hope this helps, I am going to try the test and see what happens.