Let us assume, the swept volume is 1600ml.
If all pistons were at the top, the room (C) in the chamber for gas would be:
(1600 + C) / 8 = C
funny maths...
C = 228.5ml
Let us assume the bore is 81mm, and a normal gasket is 1mm thick.
Volume used by a normal gasket is:
(3.14 x 81^2) / 1000 = 20.612
Volume used with thin gakset is half that, = 10.306
New C = 228.5 - 10.306 = 218.194
New compression ratio =
(218.194+ 1600) / 218.194 = 8.33 : 1
I probably have this wrong. Failed maths...