To address your concerns of possible clogging, I have in the past used two sheetmetal screws and a drill bit to test for clogging. Drill a hole before and after the suspected device (muffler, cat, pre-cat) and start car . Check for pressure differences before the device and after. if virtually no difference, then your in good shape. If a very noticeable difference is present, you probably have a clogged device. When finished, screw in the sheetmetal screws to seal, and the next time you are at a muffler shop have them remove the screws and weld the holes shut. Better sooner than later to avoid carbon monoxide leaking into passenger compartment.

Tim