Advantages - The noise, weight savings, the noise better flow
Disadvantages - None apart from the abuse you recieve on here
I ran a single for nearly 4 years on my car, both NA & FI. It will flow a lot better than a non true dual twin since the gases are just exiting straight out instead of going into a huge box at the back - When I had a twin fitted, my custom exhaust guy said that the back box was a huge limiting factor in my setup I can't really say if it made me any extra power though as everytime I got dynoed I had a different setup
It will be a lot cheaper to custom make aswell, I only paid about £350 for a brand new GReddy exhaust, although if I was buying now then I'd be waiting for the upcoming Tomei version
If I ever get another Z, it will have a single on it again (although I may refrain from cutting out the silencers this time )
Couple of vids of the various setups I used
HKS SC, Cosworth plenum, DC headers, Tomei decats, custom Y-pipe & GReddy exhaust with midpipe silencer removed
ARC intake, DC headers, Japspeed decats, Ichiba Y-pipe & GReddy exhaust with the silencer cutout
JWT popcharger, Custom HFC, Ichiba y-pipe & a GReddy exhaust with the midpipe intact
A single will be loud, when mine was supercharged it was 101db at idle & 122db at 4.5K RPM BUT it was completley unbearable, you couldn't even hear the person sat next to you talking because it was so loud
Everyone says how loud the K1 is but anyone who has heared my car in the flesh or been a passenger in my car will till you just how much louder than a K1 it is
If you do go this route then make sure you get a big back box with it, there was someone on here who attempted a single exit with a small pipe & box and it looked awful