which 5zigen are you talking about? there are 3 of them that are sold for the celica gts (border, fire, and miracle)?
also, ditto Kim...the TRD
exhaust is personally my favorite out of all the celica gts exhausts. it's lightweight (about 25 lbs), makes the best gains (+5.7 whp) for an emissions legal exhaust, and has a very nice sound to it.
anyway, the following link contains sound clips as well as dyno information for the 5zigen fireball (+2 whp) and the apexi n-1 exhaust (+3 whp)..along with some other exhausts for the gts (all have sound clips and dynos). turbomagazine: celica gts dyno cell with sound clips