Although you have apparently decided to change out the stock rack I will relate my personal experience with one as well as a friends experience with one. Matt did a cross country ride around 1988 with his Miyata 1000. His model was from 1986 and used the stock rack. Went up in BC and down into California as well as crossed the middle of the country and had no problems with it.
I still use one that came stock on the 1985 model 1000 and have had zero issues with it. Unlike Matt, mine has been thoroughly abused, crashed and thrashed on commutes and tours. It has been bent and straightened without any weld failures. It now serves duty on a grocery getter and shows no signs of giving up the ghost. They are good lightweight racks.
Never have overloaded the rack by keeping weight around 30 pounds, which may have helped with the lifespan of the product. When new I think the rep recommended no more than 40 pounds.