metman,
By positive slope relationship I meant that the higher the yield strength, the higher the hardness. In addition, there is a pretty strong correlation of this by the high R-squared value.
As for the selection of the 80ksi yield material, I can't seem to find a specific reason why that material was chosen to begin with. Perhaps not knowing any better coupled with the tiny price increase at the time was worth the insurance it provided someone here?!?
As for the increased thickness point you made, we have decided to go ahead and make that upgrade, since that is a small enough weight penalty. So in the end, our final decision is to go ahead with the 50ksi yield material and increase the thickness. Thank you all for your collective wisdom and help. I hope I can return the favor one day.
afarian