I got 8.12 but I allowed ever so slightly different figures for gasket bore and thickness. If you _hypothetically_ machined the block deck to get 0 deck height, that alone would bump it up to 10.9:1 . Taking 3mm off the block deck is a lot, I'd be reluctant to take that much off unless it was inspected and would safely take it. Then the issue might arise that if you went to the next piston oversize down the track that it might sit higher up and far enough above the deck to make it unworkable.

If that isn't a drama, then the 3mm or so off the block will also give you some quench area (making it less detonation prone) .

If you only took 1.5mm off the deck you'd have around 9.3:1 but the gap between the outer edge of the piston (the flat bit outside the dish) would be too far down from the head surface to give worthwhile quench, so it would be a little closer to detonation (relatively).

I suppose it'd really depend on teh cam choice too. If you stick with a very short/stock cam, then the comp ratio has to be a little lower. If you ran a bigger cam, more duration and overlap, you pretty much 'must' run more compression to get it to work well (esp the lower rpms below the cam sweet-spot)