I know what you mean about trolling too fast with the new motor, my 50 hp 4 stroke idles at 800 rpm and pushes my boat at about 3.8 mph.
I have a drift sock I use sometimes, but usually just use the bowmount for slow trolling.
I havn't heard many good things about the trolling brakes/plates... mostly they seem to be something else to get in the way or knock off on something... but you may have better luck with them.