What would the effect have been with less advance and less fuel? Also, your pump gas is what (north american system) octane?
I used to have a Tec-II in my NA EJ25 and I found that there was a good plateau where leaning it out and retarding the ignition resulted in approx the same amount of power. Yes, NA is a different ballgame but from what I'm told the same principles apply in a turbo application, just that you'd never run a turbo at 13.2:1 under boost.
I have a stock STi ECU and no adjustments yet, but I've run 91 octane with a very noticeable drop in power. At stock boost Sunoco Ultra 94 runs really well. Has never knocked or pinged on me and I've cranked the boost as high as 20 PSI (stock is 15.5 and the boost cut is at 22).
The WRX I sat in was running 10.6 in stock form. The unichip tuner leaned it out to about 11.6 as well as advancing the ignition by a few degrees. The power increase was very noticeable and the more he leaned it out the better it went. All this and the EGTs were staying quite low.
Is it possible that this large discrepancy is coming from the specific mods on the car you were tuning where for some odd reason it likes to be run really rich?
It's possible that my tired recollection doesn't have the numbers right but it just strikes me as really rich...