Yeah, like jfrabat said, don't dismiss the I4 so quickly. I have one and I've found that you can get a lot more power out of it for very little money.
I swapped in a 4.0L TB and made myself an intake with a cone filter. I spent about $100 total for all the parts and tools to do these two mods. After doing them, my throttle response got a lot better and there is a noticeable improvement in available power.
That's not even mentioning the FoMoCo injector and e-fan swaps, which will both cost me about another $100 for everything, but will give me an even more noticeable performance increase.