There are some here that have put on the 3" downpipe, but as far as I am aware it is not necessary since the stock downpipe handles plenty of flow. I would say that others could offer better input on this or do some extensive searching, since I believe there are some discussion on this subject already. The hi-flow cat is a recommended mod though and I have not heard of anyone losing bottom end with that.
I am not aware of any company that has off-the-shelf PEMs for our cars. Most here have either had them done locally, done them personally, or used a company like gtpowderworks. Andrew just had a tough time with them, but it is my understanding that their product has usually been pretty good and reasonable. You must not want them in a hurry though, as it usually takes months for them to finish a set for you and you have to provide the cores up front in most cases.
I suspect that there are other options but cannot think of any others at this time.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s