Yeppers. A lot of the US master system games are just extra EU stock. Sonic was nothing but EU stock, they just lazily slapped some UPC stickers over the EU barcodes. Due to the shared libraries, I know several SMS full set collectors who actually say that the Master System only has two regions. The Asian countries and then everyone else. Thus, to have a full set you have to have all of the Brazil, EU, releases, etc. My definition does not go this far though.
But the bottom line is, I find it hard to even consider the US Sonic with the UPC a variant. Most people, myself included, tend to think most copies sold have fake UPC stickers anyway.