Jordo's right. It doesn't matter who did the tune, at WOT, the ECU is in open loop mode which ignores the O2 sensor(s). It reads the MAF signal and has an internal look-up table to determine how long the injectors are to be open at a given rpm. It's up to the tuner who is manipulating this table to ensure that adequate fuel is present in each cell of the MAF load vs rpm table.
Similarly, there is a second MAF load vs rpm table for ignition timing that the tuner has the ability to adjust.
If you set either of these incorrectly, the car has the potential to detonate either from running lean or from having too aggressive ignition timing.
Also, a good, conservative tune should be able to compensate for slight variations in flow to through the engine. Increased flow from lets say, an exhaust should regester as increased flow through the MAF, which should in turn instruct the injectors to stay open longer. It won't be perfect, but it seems odd that headers alone would move you from conservative/safe to instant failure. I bet the original tune was borderline from the get. Maybe not with respect to AFR, but possibly with timing which is much more challenging to optimize.
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2012 Mercedes C63 AMG P31 Sedan, ROW, aFe, Eurocharged Dyno Tune. 438WHP/415WTQ
2010 SOLD: 40th Anniversary 370z
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