What do you mean by a throttle body spacer? How would this help? It sounds like another gimiky part to me.please take the approach that i will get shot on here for saying but its a 5.0 treat it like one and it will respond (proof in exhaust and intakes so far). that being said a simple throttle body spacer should net a nice little gain and make noticeable differences!
My vote was for tuning as i think this would be the most popular choice for people wanting to change the performance characteristic of their car. Its the easiest thing to do (with the right parts) and the least time consuming and cheapest. I think it would have a much broader appeal and be more of a money maker.