You can ask them to rolling road it before and after so that you can see what it's done. But I guess you'd really want to take it somewhere else for a more independent view, in fact, you'd probably want to take it to 3 or 4 to get an average as I understand that there can be some variance in how it's set up.
IMHO the best test was what me and Matt did, which as you've probably already seen somewhere on here, was to put the modded car next to a standard car on an open stretch of road and see if there's any noticeable difference. We found the answer to be a resounding No.
DMS are good chaps and I'd recommend them on service, but not on the results of the remapping.