I have a magnex on my classic and its perfect for what I want. Nice burbble, but not silly boyracer loud. Only have center decat though, as I cant be bothered with the hassle for ommissions for MOTs, and in falkirk they quite often have roadside tests with VOSA and the police.
If you are running standard, I would avoid non standard air filters or dump valves. Just seems to cause more problems than anything else.
Dave