Offshoreonly.com - High Speed Steering Question

I recently installed a new set of 1.50 B1's replacing my old 1.36's. My old 1.36's had half skegs on the bottom (don't ask). The new drives obviously have the full skegs. I am now running a larger prop, thus a faster top speed. My question. The boat is a 91 Apache 28. I have a Latham external hydralic set-up, however internally, I have the old style power assisted pump, with the Teleflex cable. At higher speeds, the boat is scary unstable, a noticeable difference from before. I did notice some excess play where the the cable meets the rear assisted cylinder at the valve as well as the steering tube assembly (the aluminum tube between the valve and the tiller arm) is completely gone. As I move the steering wheel, I have about 6' of play from left to right. I can also, see the cable at the back of the boat excessively move when turning the wheel (this even before it connects to the valve assembly). Any recommended solutions? Can this system be upgraded to the new quicksilver assisted cylinder with little motification? If I replace the entire power assisted unit in the back as well as the cable, would this help. What about hydraulic from the helm back, and how much would just that piece cost? Are there any aftermarket, lower cost units? Any help would be appreciated.