I’ve followed your "where are the old IOR boats" with interest because four of my five circumnavigations have been on IOR boats, including two roundings of Cape Horn, one of which was the first solo by an American and was part of what was then the fastest solo circumnavigation in a monohull.
The boats were EGREGIOUS, an Ericson 37 One Tonner pictured above; RESURGAM, an S&S designed She 36 3/4 tonner; and THE HAWKE OF TUONELA, my present boat, a Heritage One Ton.
EGREGIOUS may have been the first boat with a modern fin keel/spade rudder hull to go around Cape Horn, though I can not state that as a fact.
Demonstrably these boats are seaworthy. In them I have spent well over a year in the Southern Ocean and survived Force 12 conditions more than a dozen times. Their reputations for being squirrelly downwind may be based on racing conditions when they are being pressed hard under spinnaker, but all my boats have been easily controlled by either Aries or Monitor wind-vanes, even under spinnaker, though I don’t keep them up in much over twenty knots of wind.
I do not love IOR boats, They are just what I could afford. The most expensive was RESURGAM, which cost me $35,000 used in 1983. Not bad for a Cape Horner.