For a mass produced item the stock 3SGE headers are not bad, but in several design features they are not as good as hand made headers such as the Fujitsubo Super-x.
Compared to the stock headers, these will give a small but noticeable gain in mid range torque if the rest of the system is free flowing. I currently have a set on my Gen2 NA.
Aftermarket cats:
Be aware that not all high flow cats are of equal quality, and although they are expensive I found that a
Metal Matrix 200 cell metal cat worked best when I replaced my stock 20 YO cat.
I would recommend a 200 over a 100 that could be too
LOUD.
I currently use a Remus high flow catback along with Fujitsubo Super-X headers and the Metal Matrix 200, a great combo!!
BTW, check out:
http://www.mr2australia.com/mr2play/tm.aspx?m=109176 This set that I now have for sale was on my car for several months, but I recently bought another set to get ceramic coated and I have now fitted those to the car. (Don't need two sets lol)