James has it right. You'd probably see similar performance gains fron a decent port/polish job on the aluminum upper though.
As far as modding your throttle body, you really can't do much. You can cut the throttle body shaft in half lengthwise, so that air coming in has less of a restriction, but you'd only gain a couple of mms. You could have it bored/honed a little too, but adding a couple of mms really won't give you a noticeable gain. The only way you'll get big gains, would be to switch to something like Tom Morana's twin throttle body intake setup, or his box intake. Both of those require a new MAF, and a retune, but could net you 20hp or so according to his website. Plus whatever you gain with the tune.