Whenever the subject of plugs comes up I like to refer to http://www.modeltechnics.com/glowplug/firepower.html because that lists actual tests on various engines with different fuels and shows the affect it has when going from a very cold to a hot plug. In most cases performance is acceptable on any plug but there are advantages to using the right plug if you're willing to experiment a little.

For Dar's benefit, it even shows that he's correct in saying that adjusting the mixture has an effect on the ignition timing because the colder plug always can be set leaner for peak revs. I'll admit I'd reserved my opinion on this until I had another look at the column for needle settings on that site just a moment ago. There you go, I'm willing to learn

As for why OS (and I believe Enya) plugs are an intermediate length, I have no idea other than that a plug length is best if it ends flush with the combustion chamber so if they make their engines so that only their own plug fits correctly...