again, my main point is a real sump is all just water and nothing else in it, and its main job is to help maintain the water volume of the DT. so the faster your sump pump refills the DT, the better it is doing its job.
my second point is that filtration can be discussed completely without using the word "sump." as filtration took place centuries before aquariums used a sump.
so when you combine a sump, overflow and filter it gets complicated. your return pump is returning all this good stuff to the DT, such as pods, treated, heated, 1.026 saltwater and top off pure H20, etc. all this goodness theoretically mixes in the DT (eventually), and the incoming volume pushes out dirty DT water back into the filter sump to be processed. so you want it fast in to the DT but slow out of the DT all from the same pump? how?
during power failures you see just how dependent the DT is on the equipment in the sump. the DT temp & O2 levels drops quickly. when the power comes back on, you cant get that sump water recirculating in the DT fast enough.
i used to have 2 apexes: one on my DT, and one for the filter sump (I had a 3rd on my fuge/mixing station too, LOL). so I monitored temp, pH, ORP in each. I was surprised to see the sump was different in all parameters than the DT. I even switched probes and the reading were different, consistently. The sump temp was 1-2' colder during the day and 1-2' warmer during lights out, the pH was always lower especially when the DT lights were on, the ORP was like 100 lower than the DT. I considered that bad, since the goal was to maintain DT stability. The only way I could get sump parameters as close as possible to the DT was to run a very high flow sump, like 4000 g/hr (20x turnover). as sklywag states above, that rate was not compatible with filtration or a fuge, so I setup above the tank fuge and external recirculating skimmer. I stopped compromising and ran each supporting tank optimally as I could.