JigSawMan, I'm glad I could help you out. I haven't started modifying my Trep as I'm working on rebuilding a '75 Vette. One thing I should mention. As you probably know, opening up the exhaust will build more horsepower, but it will decrease back-pressure. This will cause the torque to move higher up the RPM range. This means that off the line you car will be less responsive, but you will have a higher top end speed. To counter this you will need to vastly increase the amount of air/fuel you put in to regain the low end torque. If you want to increase the horsepower and keep the torque you might want to start off with a more open muffler, put on exhaust manifold heat wrap and duct some air from the front of the car to the airbox (or "Swiss cheese" the side of the airbox). This is a good place to start because you can always add the dual exhaust at a later time if you still want more horsepower. Since nobody seems to make performance parts for the Trep you'll have to fabricate and adapt parts to do this (I'll post any part info that I come across), but don't let me discourage you from going with a true dual (Cat-back) exhaust. Talk to your exhaust shop and let them know what you want to get out of you car (maybe a true dual but using 2.5 inch exhaust pipes to keep the low end torque).
Ejlarson, you are right, there isn't much available for the Trep but also keep your eyes open for 300M products. If you want to get a "Cat-back" system find yourself a good exhaust shop that can bend and weld the pipes for you. Hopefully sometime in the future someone will come out with products for us, but even if they don't the Trep is still a styling car that can hold it's own against most cars (OK the Z06 Vette and the Viper will still kick our butts, but I've blown most 4.6L Mustangs away at the light and at the 1/4 mile mark!!)
Good luck JigSawMan and ejlarson!! Keep us informed on your progress and if you do make the exhaust change, do a Dyno pull and tell us what you get.