Mike Sabol said:
But... if your portafilter ears are too worn or your gaskets are too thin you will excacerbate the problem.
Technically, the former wont affect headspace, but the latter definitely will. If the ears are worn, the distance between the bottom of the ear and the top of the dispersion screen remains the same. What changes is where the edge of the basket mates with the gasket. Over-worn ears will combine with an over-worn gasket to start leaking. If either are over-worn, it's not too bad, only when both show signs of wear are you in trouble.
If any case, the 8.8mm is what works well for my Astoria on the stock PF's. And the gaskets should be replaced about every 6 mo.s in a hoping shop. And, the PF shouldn't be cranked. As they get older, they start to get stiff and small, and it takes more and more energy to seal the basket. When gaskets are replaced, it's a good idea to remind everyone that they've been cranking too far and too hard on too old gaskets for too long, and that they should relax a bit and be more gentle on the new gaskets. Cranking on new gaskets 'ages' them faster than any other wear factor.