I have not sent samples out to an independent lab for metallurgical testing. Lucky for us I'm making $19.00 targets to shoot at and rust away in a field and not space shuttle re-entry plates.
Yes, I suppose technically, molecularly speaking, there is some degradation to the properties of the steel edge that is inherent to the process of plasma cutting. It's the nature of the beast. I will go out on a limb here and guess that 95% of targets such as these are made with the plasma torch process as its fast, accurate enough, and cheap. Water jet is a manufacture possibility I suppose, but who wants to pay $90 for one of these? Or heat treating post cut? Again, the cost...
Now with that said, will it have a bearing on it being repeatedly hit with .223 and .308? If so, I haven't seen it yet, nor have there been any complaints from those who have purchased these from my shop and have found them to be up to the task.