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I wouldn't be phased by it that's for sure. Parfocal is a little overrated in my opinion. Even with filters of very precise thickness and a well corrected instrument that focuses all wavelengths at a single focal point, you'll still find a shift between the different 'parfocal' filters. Luckily, these shifts are typically within the tolerance of most optical systems critical focus zone. The only exception to this is if you have a fast instrument with a tight critical focus zone of maybe 25 micron or less, you run the risk that the shift pushes beyond the zone and focus becomes soft.
All this quickly becomes a moot point when you've got software automatically doing a focus routine post filter changes, or even better have filter offsets configured so that the focus automatically moves a set amount for a given filter in the light path to ensure focus is within the zone again.
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