Ah, that'll not help then! 
have you had it custom made...
or is it an off the shelf exhaust??
if custom made have you asked to keep the middle boxes? or let them make it themselves....
because on my 2.5cdx i removed them & yeah nice & throaty but seemed to loose mid range grunt & with less back pressure did drink the fuel alot more on the motorway....
some "powerflow" regestered outlets know exsactly what there doing but "some" havent a rather clue!!
the old "louder = more power = faster" approch to making an exhaust....
any idea of design?
duel pipes threw system?
bore size?
Y pipe into a single system by cats?
just a silencer?
2 mid boxes?
Or ive just realised are you thinking of having one fitted or made up??
if made right it should be less restrictive give u a slight bhp gain of 1-2bhp & the engine doesnt have to work as hard to blow gasses threw system so be slightly better on fuel, but if made wrong has an opposite effect & without enough backpressure sucks the fuel threw engine...
best option would be buy a stainless off the shelf exhaust thats been proven & tested for thoes exsact problums...
"magniflow" "serbring" "janspeed" all exspensive at around £400 but worth it tho
