I bought a 2005 Sportster 883 already fitted with SE silencers?? but still using the standard air filter- should the carb have been re-jetted and would it need jetting again if a SE filter is installed?

I would say "Yes" as Harley has been shipping their bike jetted as lean as possible to satisfy the EPA or other regulatory agencies in the country of destination. Anything you change anything on a motorcycle, you effect the entire system. Since the SE free flow mufflers were added, the bike could probably benefit from a little richer jet. If you change the air filter to a less restrictive one, that will probably lean the mixture even more.

One trick that I've been doing for years is to remove the carb and drill out the anti-tamper plug so that I can get to the idle mixture screw. Then go up one size on the slow jet. If it's a #42, go up to a #45. Then, take the needle out of the slide and install a 3mm flat washer that is about 0.020" thick (0.5mm) onto the long end of the needle and reinstall it. This will RAISE the needle just a bit to richen up the midrange just a bit. Put the carb back on and adjust the idle mixture for the highest smoothest idle and reset the idle speed. Works like a charm. I'd also replace the carb seal where it goes into the the intake to prevent vacuum leaks.

Good Luck
Steve