The towing speed limit in CA has nothing to do with speed rating of tires. And that speed limit, in my travels, is rarely enforced.
Despite all the dire predictions of death and dismemberment, I don't ever remember seeing or hearing of a case of a major accident causing catastrophic loss of life and limb attributed to towing over the speed rating of an ST tire. I'm not saying you should do it, but it certainly isn't as dangerous as some of the posters here will have you believe.
And no, a tread separation that causes damage to an individuals personal property, would not in my book be classified as a catastropic failure.