Larger gap, generally means more power. The idea behind a spark is to generate more heat in small area of the cylinder. The larger the gap, the more area you heat and the better the flame kernel, that's a general rule and there is a limit to that. However when the gap increases it requires to more power to jump that gap. Also as compression increases so does the power requirments.

Part of the function of a plug is to help dissipate heat from the cylinders. If you ran high boost you could benefit from a colder plug since more heat will be drawn from the cylinder and you decrease chances of predetonation. However the flip side to that, is colder plugs take longer to get up to a self cleaning tempature. If the plugs never get up to temp, you in theory increase the chance of carbon fowling since there are large deposits on the plug. It's kind of a tight line, and that's why I suggest staying stock, unless you run >20 psi..

________________________________________
The BMW M Roadster, if you have the means I highly suggest you pick one up. It's so choice....