Any performance muffler is going to be louder and have some "drone" to it, no matter which brand you buy. I went with the Magnaflow 12580 after learning about it through Frontier and Xterra sites. It's about the same length and width as the factory muffler, but not as tall. It's a 2-in and 1-out design, so it eliminates the factory "Y" and there will need to be some pipe fabrication to do that. I left the rear pipe and post muffler installed. I also went with Doug Thorley shorty heads and mid-pipes, but kept the stock, upstream converters.
Gas mileage and performance pretty much stayed the same; it wasn't until I did some intake mods and an ECM re-tune that I really felt a difference in performance. It takes about 500-1000 miles for the muffler to really break-in; I didn't like the sound at all at first, but it's okay, now. At idle she sounds good and at WOT, she sounds great! It's loud at WOT. At light to medium acceleration there is a noticeable "drone." It's not "raspy" like a "ricer," but it also doesn't sound as good as a V8 would. VQ engines with performance exhaust have their own sort of sound and you'll either like it or hate it. If you want quiet exhaust, I suggest you leave it as is. Here's a link to what I did to my 06:
http://www.thenissanpath.com/viewtopic.php?t=12841