If David Vizard's input is of any value on this subject, here's something I found when he was testing a different brand of plugs than Autolite:

"Up to this point, an Autolite plug had consistently out-powered everything else we tried... The baseline Autolite plugs used in the test produced two more horsepower (but sometimes more) than most other plugs we tested. However, plug testing can reveal some unexpected results inasmuch as one or two brands that work perfectly in a similar engine don’t pass muster and could be as much as 10 hp down."