After finally getting EcuFlash working under VMWare on my Linux laptop, I uploaded a new ECU image with a few fairly simple responsiveness improvements (remember, we haven't added any performance modifications to the car at all yet). I'm pretty impressed so far; the shuddering that we used to experience as soon as you got on the throttle is completely gone, and power delivery is as smooth as any other turbo car I've driven. That's not to say it's dramatically faster than before; it's targetting 16psi of boost now instead of 13, so there's a marginal improvement there, but the biggest improvement is that the car is much more predictable. In day-to-day traffic, I know exactly what it will do when I stomp on the gas to pass someone; previously, I'd have to wait for the bucking to stop before I'd know whether my attempted lane change was going to get there in time.
For those looking to do something like this, you'll need a few things. First, you need to hop over to OpenECU; they've become somewhat of a clearinghouse for information about flashing WRX and Evo ECUs. Next, you'll need an OBD-II cable; Colby Boles (the administrator of OpenECU.org) sells cables and reflashing connectors (you'll need both, although the reflashing connector just jumpers a couple of pins on a connector under the dash, and any old wire will do the job) through his Tactrix website. Finally, you'll want to see what other tuners are doing; on the Open Source ECU Roms website, tuners share custom ROM images, and with tools like Enginuity, you can modify and distribute your own.