It does sound like you could use a more powerful tackle on the mainsheet - rope burns suck however you look at it.
But you really should have confidence in your steering gear, and if you're that concerned about the cable, you should have it properly inspected/replaced for peace of mind. In good shape it should be able to withstand any loads you might normally subject it to.
Nothing worse than always being ill-at-ease because you're unsure of your boat and/or its gear when the going gets going - there's usually already enough anxiety to go around without those concerns by then.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"