I have a conservative job. Sometimes I take clients to lunch/dinner in my car. I want increased performance but I do not want to offend some clients with a performance package which is too loud.
I also don't want to void my manufacturer's warranty.
After seeing your other post, I switch my recommendation to the Infiniti Motorsports Helmholtz axle-back exhaust. It will sound much better than stock, but it will still be quiet and subtle.
The performance bottleneck will be the lower downpipes and the Y-pipe/mid-pipe; however, upgrading each of those will increase noise a good bit--enough that I would not want to take clients around.
Maybe (likely) the Fast Intentions cat-back exhaust with the larger mufflers and resonated X-pipe is still quiet enough--likely is, but I have not compared it side-by-side with my setup (which is a full replacement of the downpipes and Y-pipe/mid-pipe.
Negligible performance improvement. The primary bottleneck is the lower downpipes, next is either the Y-pipe/mid-pipe or the axle-back portion--I have not seen dyno results of a tune on a stock system compared with only lower downpipes to narrow down the impact of the Y-pipe/mid-pipe and OEM axle-back. Without a tune or even a JB4, no difference will be noticeable and the bottlenecks unimportant.