flowmasters i've never personally seen or heard but i think it has been done. i wouldn't know how loud.
a y-pipe alone will add a considerable amount of HP even without doing any other exhaust work. check out
www.motorvate.ca for details.
a test pipe is almost a high flow cat, but not exactly. it replaces your cat. it's basically a straight pipe. it is for racing only and is not street legal though a lot of people run with them anyway.
and you are correct about the maxima exhaust layout...
you don't ever need to replace the headers, it's pointless. just an FYI.