my guess in reality is the type of pipe is causing restriction. just going with straight pvc will fix a lot of the issue. Making your drain a full syphon will make it quiet and allow greater flow rates. Just dont go much ofver 600gph as thats all your emergency drain is going to be capable of. This is how I have my tank set up I could probably double my return pump volume and it would handle it but my emergency drain would get overwhelmed in a blockage situation. My current return pump is rated at 1600gph its running at about 60% but because of head pressure its probably only putting out around 500gph.