On the subject of filter socks. You may not need them at all. If your running a decent skimmer it will pick up most of the fine particulates that a filter sock does. In my own system, I no longer use filter socks unless I need to remove some sort of suspended matter. I do need to clean the skimmer more often, but it's a lot easier for me to do that.
Try your system without the filter socks for a couple of weeks and see if you need them or not. If you don't need them you have saved yourself an expensive sump upgrade.