a stock turbo won't last long with little back pressure unless it was meant to run with no cat (ie grales, themas etc), a proper rebuild should allow any turbo to run with no cat. The problem is with the seals, cat versions have diferent seal specs which turn out cheaper turbos and savings for the manufacturer, diessel turbos use the same seals....
Most new turbos use positive seals wich can cope with no back pressure...it's still not a good ideia (just because you have a bullet proof vest doesn't mean you can get shot
)