Before I get flamed..
The reason most of the "turbo crew", inparticular those in North America, run such rediculously low AFR's, is because they are insistent on running high compression engines (anything over about 8.5:1). They have to saturate the engine with fuel, which lowers the cylinder temperature, which helps prevent detonation. It's as simple as that.
If you run a low enough static compression ratio, say 7.5:1 - 8.5:1 depending on the engine, you can use a better AFR and also far superior spark timing (more advance = lower your EGT's). It all works together. You also get serious gains in torque. It's a common misconception that "low" CR's make your car "laggy". When you have a 7.5:1 zetec making 300 lb-ft at 2500rpm with a relatively large T04b turbocharger, you can't really call that laggy. It's all down to knowing what you're doing and understanding the physics behind it. Fortunately, engines are constantly getting better and more modern turbocharged engines are coming out with high compression ratios and are less prone to detonation - however, even these have relatively crap torque curves (eg, audi 1.8T, subaru, evo etc.) - they all peak then drop off as timing has to be pulled. But these are OEM cars that are used 95% of the time from 1000-4000rpm and need to be the most responsive there.. That's the constraints of manufacturing..
When mapping engines on the dyno (engine dyno), lets say your top line is 2 bar boost.. At around 2000rpm, we'll be at 12.8:1. We'll interpolate this down to about 12.5:1 by 6000rpm.. Then 6000-7500 we'll drop it to 12.2:1. This extra fuel just cools things down enough to prolonge engine life at the very top. It's not a bad idea and it doesn't cost you much torque.
Also, another misconception is observing the AFR's of OEM cars with catalytic converters. You take your average modern car, put a wide band on it and take a drive up the street. Under WOT conditions, you may see very rich AFR's, even as low as 10:1!! There is a reason for this. Catalytic converters are designed to work at a particular temperature, in this instance 750C. When cars are mapped by manufactuers, each load site is mapped to a couple of main criteria. These are 1) Until peak torque is achieved and 2) Until cat temperature = 750C. So basically they go to a load site, set the fuelling and ignition to the maximum for torque, and then add fuel in until the cat temp sits at 750C or slightly below. If you know anything about how the burn works inside an engine and anything about emissions you'll soon start to realise that catalytic converters are not doing the environment much good at all (under mid-full throttle)..
You can instantly gain power on a cat car by a) removing the cat and b) taking out the additional fuel they had to add in..
I know some of the above is slightly unrelated.. but it's all relative in the end!
after re-reading the thread, I realized my '14.7 conclusion' was wrong - no one suggested anything to that effect. Once I get the MSII in, I'll tune around that low 13/high 12 range; then I'll play with the timing - and eventually see if I can run a *touch* leaner with less timing and make similar power......