Seems like a reasonable idea. Load changes with respect to gear ratio, so it's logical that you tune for the load conditions each gear will ultimately result in. You can do a LOT with per-gear tuning, including developing a REAL snow mode (low boost and pulled timing with high throttle sensitivity in low gears, and low throttle sensitivity and higher boost and more aggressive timing in higher gears).
Big power, street-tire STi's could even benefit by mitigation of wheelspin off the line without sacrificing dynamic response or closing the throttle.
Question: How does the ECU determine gear?