Having the wheels you have in mind resized shouldn't be all that much of a problem. A good machine shop could do the work for you, and is maybe what the 4x4 shop has in mind for having the wheels resized.

Material would have to be removed from the existing centerbore area of each wheel, but that would basically amount to about 5mm of material.

One thing to keep in mind when the measurements are taken is determining if the centerbore depth will also have to be worked on. The centerbore on the wheels I had machined was enlarged from the back of the wheel, but the centerbore didn't go all the way through the wheel. There was a smaller diameter centerbore section on the front side of the wheel that couldn't be totally removed because of the design of the wheel, so we just ended up removing some material from the back side of that particular section of wheel to make room for the allen head bolts of the axle dust caps on the front axles. It wasn't really necessary for the back axles, but all four wheels still had to be machined to allow for tire/wheel rotation.