FYI: I've read reports on the forum based on Dittemore's comments that NASA was assuming a 512 mph impact, but that they were doing this as a result of doubling the estimates in order to make sure tests were run at the optimal rather than the minimal chance for damage. This would indicate they're fairly confident the debris was going at least 256 mph. Thanks. Nevertheless, I can't imagine a 260 MPH impact by a 2.5 pound piece of foam - and a glancing one at that - would cause serious damage to the shuttle.
But I'm obviously no expert. The unfortunate thing is, we'll probably never have a clear explanation for this disaster. It will be harder for the program to recover than it will be if they find a definitive cause.
I think your comments are as reasonable or more so than those who think the insulation did create the fatal damage. The only thing going in their direction, and I've actually argued the possiblity of their arguement, is the seeming failure of the left wing. Some people like you are saying the insulation/failure theorists are missing something, and that may be true. I think it's possbile the detractors of the insulation/failure theorists may be missing something as well.
I'm not convinced either camp is correct at this point. I do think both options are definately still on the table.