The
SAE (society of automotive engineers) already answered alot of these questions it seems, I checked that paper out at my libarey a few years back, so not all the info is fresh in my mind, but from what I remember serfice gap plugs like Botsh +2 and +4 platum plugs have a cooler spark due to the spark travling accrose the ceramic insulater, they also worked better with some styles of engine combustion chamber over other styles, I think larger might have been better, it sticks in my minds that big amarican V8's worked best with this style of plug, I could be wrong on this however.
With standard style of spark plugs that most people are used to the smaller the gap the hotter the spark, the draw back to this smaller gap was that less fuel/air was in the gap so the flame spred was slightly slower if the gap was to small, with to large of a gap the spark was cooler and did not ignite the fuel/air as well, higher voltage helps to a point but to high and it caused other problems in other parts of the igition system.