I think your tire guy is wrong - “tuned” has nothing to do with it. The suspension has specific objectives for ride and handling. You can go -1 (one inch smaller wheels, with appropriate tires for the same diameter) and it is done all the time for snow tires. Nobody worries about “tune”.
The 2004 XC-90, as a case in point, came with either 17 or 18 inch wheels. The suspension components are all identical. Part numbers, spring rates, bushings, all of it. No “tuning” for the bigger wheels.
The issue going smaller on the wheels is brake caliper clearance. But if Volvo makes smaller wheels for that chassis, then the caliper clearance, offset, etc. are all good.