Steve,
Much worth thought here already.
Most modern engines I've been into have a modified "squish band and bowl" type combustion chamber. This allows added gaskets to adjust compression ratio better than the original 'squish/bowl' layout, which was usually a wide, flat band with a round bowl.
That had definite advantages for ultimate power applications, but adding gaskets -shims, if you will - corrupted the effect of squooshing all the burnable fuel/air mix into the 'bowl' at the critical moment.
The recent approach makes it easier for us to play with compression ratio. Certain engines benefit from additional tailoring of the 'squish' shape, if done right by the right hands.
So, all this as just background, yes - adding a gasket or two under the head adjusts compression ratio nicely. Results are usually a more easily achieved 4/2 run, although the engines' design factors seem to urge using the low, richer 2-cycle mode. Trying to drag RPM down to the 4/2 run zone usually forfeits a lot of the power we could get, useably, from the same engine. Power rises with RPM, until the torque curve falls so far that power drops, too. (HP roughly involves torque times RPM, with some additional fiddling factors.)