Ive had the 3 magnex downpipe, de-cat and hyper muffler fitted to mine now, purrrrrrrr.....! I had the same problem, didnt realise till we started fitting it that I didnt have enough gaskets! Went round the local exhaust place and got some that where only about 2mm too small, runs fine though. Surprising just how crap the original system is, gases run into back-box at a complete right angle!
It is actually an OEM gasket that comes with the car from new.When they fitted the 3 Magnex together they re-used the same gasket.Hence you get a gasket with a 2 1/2 hole in an exhaust with a 3 hole.
The black is soot on the gasket where its been sat obstructing the passage of the exhaust gasses!! .... so in effect your new 3 is still a 2 1/2 system!! NOT GOOD! Also, with the obstruction you *could* get (though i`m not sure on this part) sound waves resonating back up the exhuast which could hamper performance even more.