It would be a worthwhile project to try other sources. SIG has piano wire, and I seem to recall Sullivan being a standard. K&S sucketh. It's always been soft, in my experience, but I've never broken it. It's been ages since I've done a Rockwell test, but I could sure get it done. A couple of 2" samples (straight) would do...let's say 1/8" dia., with no burrs on either end.
When I flew F1A, soft wing joiner wires were a major problem. Design it for no dihederal in the main panels, and all of a sudden, you got some you don't want, so your fin/rudder area is out of whack (too little). "Dutch Roll" might be the result. Not efficient, but very safe! One of the better solutions I've seen was using a stack of clock spring stock, inside a square tube. It was oriented to flex fore/aft, but not vertically. Perfect! But I couldn't find any such stuff. This was before PC's, of course. Yellow Pages didn't help much.

Steve
PS: You can, of course, order online from either SIG or Sullivan. In the case of Sullivan, it won't come direct, but you should be able to specify Sullivan wire only and get what you want. The order is actually (s'posedtabe) filled by a Sullivan dealer.