A little differential is a good thing, maybe 3/32" more up throw then down throw is about all that is needed. A shoulder wing airplane really does not need dihedral, the biggest gain would be to cut the wing apart and put it back together flat. Some other things to look at is the wing and stab incidence. You want then both at zero just the same as the engine thrustline. If these are not right then make them right. Once the differential is reduced, the incidence and thrustlines have been verified/fixed do a test flight with zero elevator trim. If it requires up trim ( most likely will ) then move the CG back until it no longer needs any trim. After all this I promise the inverted performance will be much improved.