Interesting article. I recall first seeing the effect in the lens design notes of the Coastal Optics 60mm, which is a lens still on my dream list. It was stated the lens was designed for about 2mm filter thickness.
From: http://diglloyd.com/articles/CoastalOpt ... notes.html
When designing the 60mm f/4 I called several people who were doing IR and UVIR camera conversions, and was told that DSLR filter thicknesses ranged from ~1mm to more than 3mm. There is no standard thickness, unfortunately. However, I had to pick something, and 2mm seemed like a reasonable compromise. It seems that this choice might be a very good match for a standard Canon 5D, since the filter pack is 1.45mm thick and the coverslip is probably ~0.5mm thick. Making a small error in filter thickness has a fairly minor effect on the aberrations, and even omitting it altogether will not cause a drastic reduction in performance. After all, any lens designed for film cameras will automatically have aberrations induced by the sensor glass when used on a digital camera, but very few people notice this effect.
I hope the mass market 3rd party manufacturers (Sigma, Tamron, Zeiss etc.) take care of this effect for any lens class they design. It would be interesting to see if MFT is an outlier in sensor thickness. For example, do Fuji X/Sony former-NEX/Canon M systems follow the 2mm-ish filter found on DSLRs, or is there a good reason for the thicker filter? Where a lens is shared between systems, this could have an additional impact. For example, in mirrorless land we seem to be seeing two groups already with the APS-C crowd lead by Sony and Fuji, vs. the MFT duo Olympus and Panasonic. The crop factor differences only emphasises the differences between them.
Fortunately for me, I don't really do adapted lenses. I did get some cheapies for MFT while I was using the system so performance was never high on the list anyway. I also have converted a Canon FD 50mm f/1.2 to EF, and even within expectations for a budget lens, its performance was a bit underwhelming. Perhaps this is an effect of the filter thickness? I could try it on my astro-mod cam (front filter pack removed) for comparison.