At a basic level, an AFR of 12.5 is optimum for making peak power from an engine, but the amount of heat generated at this level can be too much in turbo engines (or even when totally flat out for an N/A engine) so people tend to run a richer mixture (lower number) to help control the heat.

Turbo motors typically will be mid 11s when flat out, but in some cases where the engine is badly designed in terms of manifolds or similar (Focus RS being a good example Stu mentioned) you may have to dip into the 10s to keep exhaust gas temperatures at a reliable level.