I had a 2.5 cat back without muffler fitted last week on my D40 ( pics in my build thread ) , now if your looking for a note , forget it , its deathly quiet , makes no difference to the go , but does feel more responsive under foot
fuel has improved slightly as i confirmed on a 1000k trip ive just returned from , and go to a few smaller exhaust places and get prices , it will cost you 1/3 of what your mentioning here , infact mine was so inexpensive , you would freak !!!!