Assuming that the coil can generate the require voltage, what effect does increasing or decreasing plug gap have on combustion?

As usual what i say is more or less a guess.
An increased gap would probably increase the burn rate slightly, but i am more or less sure that it would help alot if the mixture is lean.

You've answered your own question.  Increased gap means increased voltage for spark/arc breakdown.  Eventually you can get enough voltage to cause insulation failure in the HV train.

Some years ago I undertook an extensive study of various spark plug designs and types. This was carried out on a single cylinder version of a modern contemporary 24 valve() 3.0 litre high performance V6 engine design.
The types of plugs that were tested were
a)    a conventional