Shorty's do not add power, and are fairly useless. The factory headers are very stout and not restrictive as others from the past have been.
LT's are the way to go if you can do it. Check your local smog laws to see if they are legal first, some crazy states like CA, suck azz.
I picked up 60+ rwhp/rwtq with LT's, CAI and a tune. It sounds like a monster.
If you want to keep it quite, keep the factory mufflers. If you want a screaming monster, loose the factory cans and add a good set of LT's.