Well, here's some thoughts on your four points
1. So far, the only filter socks that fit in my 100.3 are the waterbox ones. I've tried BRS 4" and DFS 4" so far, and neither of those fit. Not sure if its just my sump or if other waterbox owners have the same issues.
2. I liked this feature too, it's too bad they do a horrible job on the silicon. Spent hours scrapping off all the access that covered the over flow sides. Also had fun trying to scrape it from around the holes for the bulk heads. The gates also look completely scratched up. It's less noticeable with water, but still looks like a crappy job. Like I said before, they have major QC and quality issues.
3. I don't like Red Sea's either. I also don't like waterbox's as it's not the 4.7 gallons they advertise, more like 3.5 which is about 3-4 days worth for my evaporation rate. I have a Neptune cor-20 pump and the Neptune atk and there is barely enough room in the return chamber for both. I wish they made the return chamber bigger. I'm also going to have to use a different ato reservoir. Which is what I would have done had I gotten a Red Sea.
4. I liked the layout too, until I had the tank running. The baffle is too high for the intake section, so when the pump starts up, water splashes out of the sump until all the air is out of the return and drains. I'm going to have make a lid for that section. The second filter sock chamber makes a lot of noise, have to add a filter pad to quiet it down or maybe get a Flir. It doesn't seem to be able to handle anywhere near the 1000gph they recommend without being insanely noisy.
I also had to cut the plumbing for the return. The return pipe was too long for the overflow box so the T didn't line up with the pre-drilled holes for the return nozzles. I had to cut 1/4" off the return pipe for it to fit.
If I had to do it over again, I would have gone with Red Sea or IM. I think waterbox is just cranking them out regardless of quality. Definitely not comparable to the quality I've seen in the Red Sea's or Innovative Marine.