Compression ratio is determined by head thickness, and by gasket thickness.
Contrary to popular opinion, there is no need to install hardened seats in the head. The Volvo cast iron alloy is quite capable of running unleaded fuel without trouble. If you knew how many unmodified B20's are flitting around western Canada, on unleaded fuel since 1986, you would not waste your money on hardened seats. Compression ratio is determined by head thickness, and by gasket thickness. Contrary to popular opinion, there is no need to install hardened seats in the head.