ok if your not getting much diff in flow now then it sounds like to much packed into the filter. test it by removing the polishing pad and possibly some media, but if you have sludge sitting in the bottom that isnt good an makes me belive your sponges are not cleaned enough. from my experiance the coarser ones *tall ones* might need a bit of waterpressure to really get some of the gunk out then the sponges in the tray are fine and mostly catch silt. ill see if i can get a vid for a reference of my 406 for ya to see the water flow.
alright i did a video of my fully cleaned 406 with full media but no polishing pad. as others have said id do the test with a 3g or 5g bucket and time it then multiply to an hour and see what your gph is, it will vary with each tank with polishingpads,heaters,head height, etc. but i bet your right in line with where mine is. i took of the nossle to show u in the video as your lilys do basicly the same as it without the nossle*no build up of pressure to force water* just nice gentle flow. do the test with a bucket first an let us know where ur at.
hope it hleps ya