During Closed loop operation (partial throttle, most of the time) the MAF and O2 sensors will attempt to keep the car at stochiometric which is 14.7:1 by cycling back and forth from lean to rich corrections. When you go to open loop (WOT as in a Dyno run) the ECU defaults to Fuel maps programmed into it. From many dyno reports I've seen here the car does run rich. Semtech posted one with a few mods and it was still at 11.8:1. My Scangauge indicates a pretty steady rich correction number of -7 to -9% with just an exhaust which confirms this. This is why this car makes such good power improvements as you add performance parts and it leans out some. Max power in a N/A engine is made at around 12.5:1 A/F though that will vary from car to car. The ECU also seems to have a pretty wide adaptation range which is why many mods can be done without re-tuning. This is a good article on A/F ratios:
Autospeed article on A/F ratios
Rich mixtures generally cost you in Power output, but provide some Safety factors in that the charge will be cooler and the chance of detonation lessened. Leaning out a car to a certain point makes more power. Go too lean though and you face high heat and detonation issues.
On my old Corvette that I had tuning software for, you could easily see a 5% HP improvement just by adjusting it's Factory rich tuning to a better, yet still safe, A/F.