For Volvo 142E's & 1800ES , they make a thicker gasket for the exact reason you're looking for one. At 10.5-1 they will often ping on 91 octane. I do have one question on the motor, what Camshaft is in it. The compression ratio and camshaft usually go hand in hand, depending on the cam lowering the compression may have adverse results (nothing fatal just annoying)
As for the guy saying higher compression engines will implode.................that's a total pile of dog poo. A series race engines are at 14-1, the short service intervals have more to do with the high RPM's than compression ratio.
I've always stuck with 10-1 because I just don't want to be bothered with running race gas, it's availible at our local track but I like being able to fill the car at the gas station as well as the lower price.

Tom