Now I see you are two steps away from the recommended stock plug. I could see that causing a reduction in power. If those plugs stay in your engine for long, it will be interesting to see if they foul quickly, cause misfires, etc.
It's hard to compare heat ratings between plug manufacturers; they are not the same.
I'm with Phil on using the stock heat range. Unless your car is for track use only, where you are running it hard most of the time, colder plugs will just not do anything for you in a detonation reduction sense.