I had a second wheelset built up. I specced the same brand hubs (but Tiagra to 105), same brand and spread cassette (but Tiagra to Ultegra). The wheelbuilder measured the dropout to low cog distance of the existing wheel for reference, but told me the Shimano hubs were very consistent. The 10-sp shim aligned the 10 sp Ultegra cassette perfectly on the 11 speed 105 hub.
The rims were 1mm wider on the new wheels so the canti- shoes are set very close and may get a slight turn of the barrel adjuster when going to the old wheelsat but the differnce in lever travel is not significant so adjustment is not always required at present, but that will change as the pads wear.
The first ride showed some shifting noise, but a bit to tightening of the RD cable eliminated it and actually made thh original wheel set shift a bit better.