shorties are a waste, if you got the $$ for long tubes, then do it. many people have posted big gains, but it also comes with a heavy price tag. expect to pay around $1000 for the pipes and new h pipe, then installation is around 8 hours from what i have heard. at $80/hour for a shop to do it, i dont think so. if you can do it yourself, or have deep pockets, thats when it is time for the upgrade.
I have the Kooks LT headers and I love them. They are expensive though. Before my car had only a C&L CAI and Canned Predator tune. I pulled 286RWHP and 305RWTQ like that. Then I had the other mods installed, see signature. I then pulled 321RWHP and 344RWTQ. You do the math. Some of the gain was due to headers and some the X-pipe. But, it does sound mean and the low end torque is great.