Tom is the guy to go to with this kind of stuff, his username is 1turbofocus. He's kinda the ECU Guru on here.
If you'd like, we're having a meet and greet at southcenter mall this coming sunday, you're more than welcome to look at my ECU tuner! You can look through all the options and get an idea of what its capable of that way. Also, there will be a member there who has an ST with a tuned ECU, so you can ask him all about it! It definitely sounds like you've done some research on this stuff, so no noobness :P
An ECU tune should help with hanging RPMs and MPG's though. I'm sure thats not out of the question!
ROTM June '13, PNW Crew #42, SR #245, Rally Rev #02