You should generally only change the plugs to a different heat range if the current heat range is posing a problem, i.e. appears to be too hot or too cold

. Standard gap for all the current blower systems available is sufficient. With Gap, bigger is better up until the point the spark cannot make the jump. Standard gap (the gap the plugs come with) isn't a problem-- even at 285whp and 8500rpms...
Iridiums are nice for longevity but make no appreciable excess power than their cheap copper core cousins. If you're simply trying to determine the correct heat range for the car, buy the cheap copper versions (of the same brand) for testing/tuning purposes. Once the correct heat range is determined, go buy your expensive iridium plugs that will last 50-100K miles...
FWIW- I still have copper plugs in my car and she runs okee-dokee
Best,
Phil