The trouble I've found with attempts to slow down with slip or spoilers is that it's very hard to get timing right. You want to slow down while the slack is getting bigger. You don't want to be slowing down once the slack is big, or as the rope is coming out. A second delay, and you're making things worse.

A steady slip or spoilers in very rough air can be quite helpful for keeping slack from starting. Some times on long retrieve tows I put a few degrees of slip in and leave it there, which smooths things out nicely.

John Cochrane BB