The decel popping (after-firing) isn't harmful a least. Only annoying. Only back-firing is harmful (combusting on the intake side). And being a left coaster, you may be more noticeable with that noise to the po-po. That's where blocking the SAS is useful. Normally unburnt fuel that gets exhausted out the pipe stays unburnt unless the heat from the pipe ignites it, but it still requires the negative pressure wave in the pipe to draw in fresh air from the exhaust tip so it can burn. The SASs job is to ADD fresh air into the exhaust all the time to help the cat work more efficiently. With all that available fresh air, unburnt fuel from the combustion chamber will now burn in the pipe, so you'll get more of this more often unless you block it all off. Mistral and the others that make the Y-pipe should automatically provide the SAS block-off kit to customers, IMO. Or at least, they should write this as a footnote recommendation on the web page for the part and in the installation instructions.