I have/had a duplex system with a mid box about the size of a bean can and that sounded awsome to be fair. But now im going for a boxless system with just a jap rear can. With a system with a mid box you will notice a bit more mid range poke where as a boxless system will bring the power in higher in the rev range. But them with the decat it will even out the flatspots in the midrange of 1st and 2nd gear. So I would say boxless sytem with decat SHOULD give you the best power. @Meech i just been quoted- £268 for a boxless system cat back or £309 for a system with a mid box