I would keep your stock throttle body for right now. There's other upgrades that you'll benefit from more currently. When you're adding finishing touches to your GT, a throttle body will be a nice option.
I highly suggest an aftermarket stall converter
, as others have said. Depending how much you plan on racing the car or how much it's driven on the street, would change my suggestion of which size you should run. If you want a little bit of both worlds, I've found the a 3,000rpm stall is nice.
You mentioned you're looking for more sound too? I see you already have Kooks headers, but are you running a catted or O/R mid-pipe? Also, what catback or axleback are you using? I'm sure we could find the tone you're looking for with the right axleback.
Let me know if there's anything else I can help you with!