I had to remount my filters last night for the very reason of shimming. When I first had my filter wheels I did not pay much attention to the fact that some filters were thinner than others. The Baader filters are 2.4 mm thick but the LPR filter I bought was only 1.2 mm. It made quite a difference in the focus positions.
I can only infer that the shims are offered in different thicknesses to deal with this issue- so my advice is to shim as close as possible to match the thickness of all filters.
Hmm, that one I don't understand - what you mean with 'shimming'? Are you adding clear-glass filters to match the thickness? I'd expect that to reduce optical quality/performance,,,
(and I'm *really* looking forward to your filter-flip test!)