Sandblasting WILL remove some metal. A good operator with lots of skill at picking the settings and watching what they are doing can minimize this but sand is going to remove some metal regardless. For a really nice road frame where there isn't a lot of material to start with for wall thickness you really want to look at one of the other media. However if I had seen examples of the blaster's work and he had shown that he was masterful enough to do the job right then fine. A good craftsman can do amazing things with minimal damage.

Have you thought about using a paint stripper on your own to prep the frame for powder?