but you're a 99 RS-T
Why go RS ecu?
Why not do a WRX/STi ecu (non drive by wire for ease of transition if you wish, although it's not much more work) and have a turbo-ready ecu/sensors etc off the bat?
edit: doh! I just remembered about the heads...
Yah I suppose that would be an issue but you can find cheap WRX heads off a parted motor for pretty cheap!