Thats ignition timing.... The roug timinh is set by the headshims and finetuned by the glowplug. A colder plug will give a slight later ignition which can give a tiny bit higer rpm and a warmer plug will give a early ignition which will give a tiny bit more bottom power. But running a 5 in a .21 high end competition engine which does have a high compression can result in broken plugs.

Finally this is only done as correction to the outside weather.

A tiny change to the cranktiming will do more than you will get with changing ignition. Inlet and exhaust timing are the things to change to get more out of it.