From my years of reading this forum, I have come to the conclusion that there is NO exhaust mod worth the time and expense when it comes to adding HP, most are for sound.
If you want to spend really big bucks, there are some exhaust headers available from Rob at NeedsWings, but I don't think you gain too much power.

The 3 things I listed above are about all you can do to an NA to gain some torque/power, there are TONS of stuff you can do with an SRT6.
I have had all 3 on my NA for about 4 years now, no problems or complaints.

DJ ( tighed1 ) is about to get his NA back from Corky at Bell engineering after 4 years with a turbo.
We are all waiting to see how that finally turns out.