Theres more to it than just having the "perfect" Stoichiometric lambda. An N/A engine will be totally different to a force induction engine, for starters it has a higher compression ratio, the higher the ratio the higher the octane rating needs to be to keep det away. Also an N/A engine is only able to draw air in at 14.7 psi or atmospheric pressure on the intake stroke. Even then if you take into account the resistance to flow before the air even gets into the cylinder your N/A engine will only be sucking in around 12 psi of air. This is around a 10% drop in peak power! Running an AFR of 13 on a turbo engine at WOT to me is ridiculous, your already increasing boost pressure, moving away from the turbochargers efficiency envelope heating the air at this point, plus the heat of resistance to flow. Without injecting a "safe" amount of fuel cylinder pressures are going to be high, and are more likely to induce det or even headgasket failure. Every engine will respond differently to X amount of fuel and Y amount of ignition, every batch of fuel you place in your fuel tank will be different. One engine may perform well at an AFR of 12 or another at 11.6. No offence but I know people that have been a mechanic for 25 years, a mechanic is totally different from a tuner, and for this reason I wouldn't let them build me a race spec engine.