102mm or there abouts I think, performance difference none I can tell and I don't expect there to be any. Noise difference massively, if you want to hear what it sounds like then take the resonator off and go for a drive it'll heatsoak like mad (which is why the ducting is needed) but it'll sound the same. Its a very deep bellowy noise low down the range and it just sounds ace up the top end.
Its got a K&N panel filter too, that makes a difference as well.
edit:
http://www.venairsport.co.uk/product_info.php?cPath=49_78&products_id=117
Think its that stuff though it could be bigger, I worked out the diameter I needed using Pi.
Pretty sure its that stuff as the circumference of the airbox mouth is ~300mm and 102*pi=320.