850 Cylinder head Skimmed

When I have had a similar problem, I made a spacer out of 18 gauge aluminium and used two cylinder base gaskets. When guys increase their compression ratio slightly it changes the balance between compression, mixture and ignition timing. If you raise the compression ratio slightly , it has a similar effect to advancing the ignition timing or leaning-off the mixture. Most guys probably runt their motors jetted slightly too rich, so raising the comp. ratio improves power output. The same increase can probably be achieved by getting the jetting closer to the optimum. But if you do that, the weather can affect the way the motor performs.
If you raise the compression a lot, the law of diminishing returns comes into effect. You use more fuel, so theoretically you get more power. But in practice, that is not necessarily the case. There is aa 'quench effect' when you use more fuel to get more power by raising the compression ratio.
I suspect that most Commando owners find their bikes perform much better on very cold days

When I tune my motor, I do not change the ignition timing from the value I have selected to suit the fuel octane ratio, and I do not change the compression ratio. I simply jet the carbs to suit those constants. until I get best power. I have a friend who does it another way. He puts the bike on the dyno and gets the jetting right using the oxygen meter. Then while the bike is running on the dyno, he advances the ignition timing until he gets best power.