Added a bobs cav plate and a 'slight cup' to my 15hp 2s merc (10p stainless prop) last week. Before the additions I was getting a steady 18.5kts with two people with 5400 RPM on the tach. Now I'm getting around 4300 RPM on the tach (i did notice it slip to 5000RPM at WOT one or two times) and my speed is only 14-15kts. I also have a jack plate and raised the motor up about 1" from where it was before. Without the additions it was blowing out on turns (outboard at transom height, no jacking up ), and jacked up it was slipping and cavitating. Now it's not slipping or cavitating but the loss of speed and RPM is way more than I thought it would be...? it is still blowing out on turns though for a few seconds. Does it need to come out of the water MORE or less? It's only about 1" above my transom right now...the stock cav plate is about 1-2" above the top of the tunnel in my hull's plane. The outboard has the three tilt settings of 1-3. I have tried them all and settings 2 and 3 seem to be the best when on plane...setting 3 being slightly better due to the hull's running angle.