Yes you are right a decrease in head shims or an increase in compression ratio will increase torque because as the compression ratio goes higher the actual ignition timing occurs sooner. This is the same effect that happens with a colder plug except in the instance of a colder plug the ignition is slowed until a greater point of compression build occurs. however there is a point of diminishing returns where detonation occurs or engine temps can soar this is where a colder plug can also help.
Consequently the same effect occurs when you increase the head clearence. top end is enhanced as the ignition timing is retarded and occurs later. generally a hotter plug is needed to advance the ignition cycle so the timing does not occur to late in the cycle as at this point you end up overleaning the engine to get it to rev properly and the engine life will suffer dramatically.

on a 12 engine I would only advise going .004 thousanths over or under .018. so that is .014 or .021. I have run as low as .008 but this was with tremendously increased exhaust timing and to regain static compression ratio. to little and the piston can hit the head as you have to take into account rod and piston growth relative to their tempratures.

On marine engines we could get away with .003 on a 21 but we were water cooling it. that won't fly consistantly on a car engine without a special head that has a larger combustion chamber to start with.

Savant you can safetly use 30% on the stock head clearence. using an odonnel 99 plug you can go 40% If you dont overlean it the engine life will not suffer dramatically.

I should also comment that my comments with regards to plugs and head shimming are when both are used together. used alone a hotter plug will rev harder and a colder plug will make more torque.

there you have it.