I've never had a slip, but it sounds like it is pay now or pay later Charlie. Might be easier to paint over now than have to scrape off growth and paint later. I guess it depends on what shape the existing bottom paint is in?

if the boat has been trailered, it will have hard bottom paint, it can go in and out of the water, but its not as good as ablative for long term. Ablative will get hard if its left out of the water too long, we used to lightly pressure wash them before putting the big boats back in the water to get the hard top layer off so the ablative will do its job. Hard paint(trailer boat paint) can effect your performance depending on how much paint is on the hull, and how good of a paint job was done. Soft pain(ablative) will most defiantly effect your performance, we usually drop down at least two pitch sizes when putting ablative paint on. Its normally only used for big boats that stay in the water for extended amounts of time. My sea hunt was purchased from a guy that ran a diving service, he kept ablative on it. My boat is about 10 mph down from what it should run, I've been pressure washing the bottom every chance I get, once i get most teh paint off, I'm going to wet sand the bottom and try to pick that speed back up. I don't know how bad the growth is up there, but down here we have divers clean teh bottom once every 6 weeks in the off season, then from may thru September, its cleaned once a month, plus the day before you plan to run it.