I have never had any issues with cars that have bolt in silencers (mrs' car has a fart cannon I haven't yet replaced from when we bought it). Guess it depends on your wof man, and their interpretation of the rules. IIRC it reads something like 'cannot easily be adjusted' or some shit. The fact its bolted in mean it requires a tool to remove it, hence not easily adjusted.
I only got it one wof with the silencer in as it was starting to rust so decided to leave it out. Getting wof's with just the fart cannon have been fine, and my mechanic says its sweet legally, but he's a family friend so I don't know if he actually bothered to measure the dBA (I doubt it). Just don't drive it like a dick and you should be sweet, works for the mrs, that said it drones too much for my liking and muffler/fart cannon will be replaced as funds allow.