FWIW, I think you're doing the right thing, Denise.. Putting more effort on the wheel will only slow the boat down and make the rudder more likely to stall out causing you to lose control completely. I'd be worried about the rudder itself - not the cable.
A better tactic is to try and work out what is causing the excessive weather helm in the first instance. Perhaps you can't dump the main quick enough??
"Honestly, I don't know why seamen persist in getting wrecked in some of the outlandish places they do, when they can do it in a nice place like Fiji." -- John Caldwell, "Desperate Voyage"