The airbox might not make the difference....
I do know that even a small leak in the system, seems to knock it out of reglulations... i didn't replace my mesh O ring last summer when re-fitting the joint when perhaps i should have... the inspector did say it had a small minor leak and also at the back box....
might be worth trying a new O Ring...also, the fitting kit they provided with the system was a bit of a mess... the recessed spacers were not only the wrong size but not recessed either...
If that doesn't work Alison let me know and i'll post you down my spare center section which is a 'proper' oem one... as long as you promise to send it back up again..as it's worth a few quid...
would be interesting actually to compare the figures from both pipes.... bit anorakish i know...
Regards