If you were going bare floor, I would suggest putting PVC just off the floor...works really great for waste removal and water changes. We have two "poop sucker" pipes in our 300 gallon tank. We drain the tank by opening a valve. The water gravity flows out the tank. I changed out 150 gallons yesterday--would have done more, but the fish are so big they were on their sides. Here's an old post on our mechanical system. Tank has been running over 8 years with no debris to vacuum.
http://www.monsterfishkeepers.com/f...n-Pic-Thread&p=1211087&viewfull=1#post1211087
The spa filters have worked so well, that I wouldn't consider not having them. We are going with six poop suckers on the big tank with six larger spa filters. I had eleven extra holes drilled in the top of the new tank to accommodate our closed loop mechanical and biological filtration systems. I personally am not a fan of bulkheads low in the tank and prefer pipes coming from the top of the tank.
It seems that most sumps are syphon fed via some type of overflow system. Perhaps you could incorporate a "poop sucker" uptake close to the floor of the tank, with a couple of holes near the top of the uptake pipe for syphon break at a pre-determined water level. When the water level drops, the syphoning action stops at the drilled holes. This still would not be a closed loop system though. For closed loop, the water is pumped out and pumped back into the main tank. And while this might work for removing debris from the bottom, when it came to removing water for water changes, you would be limited to the predetermined low water level. The tank could be plumbed so that you can turn a valve and drain water, but why not have some extreme mechanical filtration also?
I've seen spa filters mostly used as prefilters on large reef systems. The guy I contacted when we were setting up our 300 g tank sets up and maintains reef systems. That's how we got ours. While they are only mechanical, they do an excellent job.
We are using two RTL-25s on our current system, but will use RTS-100s on our bigger system.
http://www.spapartsshop.com/R172505.aspx
Here's a fabulous reef build. You can see the RTL-like filter that is being used.
http://www.reefcentral.com/forums/showpost.php?p=16850559&postcount=809
There is only one other MFK member that I know of who uses RTL-like filters. Oftalmos uses two of them on his 765 gallon tank.
http://www.monsterfishkeepers.com/f...-My-765-gal-tank-Part-1&highlight=Mexico+City
These are the 8 uptake pipes and three return pipes going in the new tank. Even the sump tank will have a vertical pipe drawing water from the bottom of the tank and returning the water to the main tank. The plumbing will be the only "decoration" in our undecorated tank.