This is very interesting, Scott. I would be interested in hearing the type of player that you are seeing this with.
I agree with you about HI-COR for the most part. However, I do see some benefit for some players. Even on conventional irons, there is a progressive gap compression in the lower irons. If this compression is magnified by the reduced SS of a shorter shaft in the case of SL irons, these 2-4 extra yards become important. I also think that HI-COR is helpful for many players in that the higher ball speed may bring back into play a lower iron that may otherwise be unplayable or marginal.
The problem is that HI-COR is sold as a panacea and appeals to the distance fetish that golfers have (distance is to golf marketing, what sex is to marketing just about anything else). I think there are many players who are using them for the ego factor, and they aren't really accomplishing anything. In fact, the longer distances in the short irons become a problem because anything from 110 yard in -- a large portion of your shots -- become partial shots. I suspect that these are the people who are most at risk of collapsing faces, too.
So, by the sounds of things there is a sweet spot in the marketplace that can/will benefit from this additional ball speed. As you say, the lower end of this segment is defined by those who have a mid-iron swing speed of 75 MPH. The concern you have raised about the faces deforming/collapsing suggests that there is an upper end, too. I think this suggestion reflect Tom Wishon's comments about the Sterlings. He has said repeatedly that these clubs are not for everyone. On his blog, he said that he designed these primarily for players with an 8 handicap and above. I think that this market definition probably reflects a more average SS/ball-striking ability.