As they say, follow the advice and remove the plugs one at a time until you get the right response.
The plugs won't change the top rpms of the prop. They work by allowing exhaust gasses to leak out of the hub in front of the blades of the prop. This will cause the blade to slip( less resistance in air than water) and this will allow the engine to rev up faster, get into its powerband faster, and thus allow a faster hole shot.
At higher speeds the exhaust is sucked out the back of the hub and the prop runs normally.
In the old days, you used to have to drill your own ventilation holes in order to improve prop performance in reguards to hole shot. Merc has come up with the best system so far.