I might be wrong about this, but I suspect that a combination of a low torque spec to avoid stripping the threads out of the head and the different heat expansion rate of steel and alumin(i)um is going to result in the plugs working loose over time. This should become part of your regular 'nutting anf bolting' routine.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python