I agree with Auballagh on the drain situation for safety. If you search for my posts, you'll find I'm nuts about noise, so I feel the sentiment, but safety first. In my brain, if you only have one drain, you can't do what you're trying to do by limiting flow - you really need a second emergency drain just in case anything goes wrong. If something happens to your drain, you'll wind up pumping your whole sump into the aquarium and onto the floor - scary.
In order to limit this as a possibility, just drill another drain into your aquarium
Actually, If I only had one bulkhead, and I were trying to set the level in the weir to reduce noise, I'd go with a single durso stand pipe with a valve in the top to tune the air to allow minimal noise.
I agree on the pump also. Pumps lose effectiveness as they have to lift water. This is called "head loss" and it reduces the effective flow rate of the pump as it's now also working against gravity. A good rule of thumb is to take the published numbers and divide by 2 to get a more realistic flow rate.
So - the big pump is 4000 lph, and your tank is 550l, so 4000/550 is only 7.3x tank turn over on paper. If the pump actually only runs at half efficiency, you get just under 4x water flow through your sump. Most people around here would tell you that 6-10x is a good number to shoot for.
I would personally lay the filter floss on top of the sponges and let the water flow out of the first chamber onto the top of the filter floss. this would be the same way most folks run filter floss with a wet/dry or trickle tower set up.
Here's a thought - no reason the heater can't go on the floor in the intake chamber, right? Best of both worlds!
Here's another thought. What else are friends for? I would scrap the egg crate basket and bio media in favor of a piece of Matala Mat. This is a structurally mostly rigid sheet about 3ish cm thick with an internal structure containing lots of surface area which is often used in pond filtration applications as it allows for lots of water flow and is hard to plug up. It would allow you to set the foam on top and water would still flow through it and out into the pump compartment. I do virtually the same thing in a vertical configuration in my sump. I think of it as my Hamburg Matten Filter sump - your design would be similar if you did not disturb your sponges. I like it.