The TRD's hp gain is very similar to TOM's barrel. Also, the tip of TRD is very similar to our stock tip. My friend tell me he did not feel any big difference in HKS an TRD. But he said may be the sound made him feel HKS is better hp. No dyno prove don't know. In my opinion, you can't compare the Altezza (rumble engine sound)to our IS300 (slient engine sound), if you put a TRD exhaust on your IS300 can't notice much difference in sound and also looking than stock.
Don't go for the TRD. I've asked mechanics in Hk and they say TRD is just selling the name. They said it is much to quiet and not worth the price. I used to have a Remus for the IS200 which is very similar to the TRD exhaust, but it was to quite. Now I've changed to a HKS and I scare the crap outta passerby when they see a Lexus