Hey David... I got two more cents. I did the JWT/Z tube/Plenum spacer as well, (and you're right, it does seem that is the most given answer to the 'mod' question... prolly because it is relatively inexpensive, and has nice results), but I also got myself a GRedddy SP2 axle-back system. It is very easy to install, and yielded a nice deep sound without spending $1,500 on a full cat-back system. It is nice and mellow under normal driving, and has a great sound under hard acceleration, although, not too loud.
Here's a link to a pic of it:
http://www.partshippers.com/in...=1003I actually got mine, barely used, for $250 from the Classified forum. I may add a mid pipe, and high flow cat later, but for now, it is just right for me.
Hope this helps!