Daz,

Without having the gasket I can't tell you why it failed, but head clamping does sound suspect.

Did all 6 long studs click off at 90lb f ft, what did you use/method for the outer 4, and what was the crack off torque of both the studs and bolts when you took the head off?

I personally would not advise the cometic gasket. Stick to the genuine mountune item. It may seem expensive, but in comparison the cometic is absolutely day light robbery for the method of construction used so is in fact worse value than the mountune item!! Better to pay too much for a quality item than quite a lot for a shite item.