Wow, your second description is really different than your first description. That is some bright sunshine above your backstop for a day when it is raining.
Your bags of rubber will fail and your secondardy backstop will see a lot of use, unless you do a lot of maintenance and remove damaged bags of rubber and restack between classes.
This is especially true if you are getting 16" of penetration on a sacrificial 24" barrier that degrades with each shot. Because you use bags to hold your rubber fill in place, holes in the bags will produce leaks that will form into tunnels. These tunnels will not infill with rubber from above unless those bags are significantly damaged. You will lose fill behind your target centers such that your 24" of barrier will diminish to fewer and fewer inches until you have a nice bore hole through.
Your target wall does not allow for much movement of the targets vertically or laterally and so your impact areas are going to be very concentrated, hence exacerbating the problem. You have a fairly large backstop area, but your primary impact areas will be fairly small.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011