Many of us on the forum with sand filters routinely (after backwashing) add a little DE to our filters. It seems to help the filter catch more stuff. You only add enough to make the pressure on your filter rise by 1 psi. Premix it with a little water before pouring it in the skimmer. It will usually take 20-30 minutes for the pressure to rise, so wait a bit after the initial dose before deciding if you need to add a little more. It might help clear your pool faster.

Also, normally I wouldn't recommend shocking the chlorine up that high with a cya of only 40, but since the original poster said that the liner was going to be replaced in the spring anyways, in this case it is probably fine to do so.