A test pipe won't help your performance any. The factory cat on these is pretty good, unless it's clogged. If yours is clogged, you can buy a high flow cat for about $100.
As for your 3rd gen Maxima cat-back, you can get a complete bolt-on set up from Warpspeed and you can install it yourself.
A muffler shop can usually custom fab something for a couple hundred. It depends on the pipe you want and the muffler.
A cat-back gets you very little in performance gains on these cars. If yours is rusted, then you should replace it. Otherwise, start with a y-pipe.
The biggest gain in performance is with an aftermarket y-pipe. Most people buy the pipe and sometimes all the way back from Warpspeed.
http://www.warpspeedperformance.com
I have a GReddy cat-back and a Cattman y-pipe. I did the y-pipe first and I could actually notice the gains. The cat back had little to no effect. This is pretty much the same on VE and VG engines.
My only advice on an exhaust for these cars is to get a resonator. Without it, the inside is annoying and loud.