A launch turning in about 6-7 times its length at speed would probably be deeply uncomfortable for the crew. Overdoing speed when turning tends to result in either the boat leaning out, and, in extremes, flipping as a result, or digging in the inside side.
Speed and behaviour in models do match the real thing very closely, that's why test tanks have been so useful over the past centuries. What passes many modellers by is that scale applies differently to different parameters. Distances are linear, areas use the square root of the scale, as do speeds, and volumes and power the cube root. Real world water doesn't scale, it remains water.
Once the optimum rudder setup has been found, that's it. If it turns adequately at low speed, any high speed improvements will need to come from motor control and will probably involve differential power, either manually via two sticks of automating the process using a mixer, either in the boat or in the transmitter. Adding bits to the rudder will create extra drag which won't matter at low speed, but will prevent the high speed turning problem by preventing the high speed in the first place.