It is possible depending on the drop height and other variables and only with a siphon. Without a full siphon drain, it would not be possible. I am skeptical if a 1" ID pipe turned down 1/2 way can handle all that a 7000 can flow at a normal head height of around 5-6' after running some quick calculations But if you say it is doing as such, who am I to argue.
On the subject of the wb5000, You can see the flow in this pic and no other pumps were on at this time which was why I was trying to point out that it is a lot of pump, more than most realize. Also, I am running a full siphon drain through a 1.5" pipe with a gate valve turned down around 1/2 way. That is a lot of surface turbulence even 25" away from the outlets of the locline. My 5000 is dedicated to the main tank in this setup only since I run a full siphon with emergency drain.
If someone was to divert a 7000 into say a fuge or ats, or a chiller and maybe some reactors, then yeah a 7000 would not be a bad choice. But say if it was just for a display tank and say some reactors, it is way to much pump. I just hate people purchasing more pump than they need as it is just a pure waste of electricity. But if they plan to upgrade or add other systems into the mix, then by all means go for it!
On the other side of things, If one is to run a siphon drain like you and I, I DO NOT recommend trying to run other items off the main pump unless it uses a micro amount of water like a ca reactor as you will always be fighting system / siphon balance. Use a dedicated pump only and a secondary pump to power other tanks, systems or reactors.
In hind sight, I might have gone with an even smaller return pump than the 5000 for my siphon system and ran a smaller primary drain line as with most systems like mine, the purpose of the return pump is NOT to provide flow for the main tank, it is water turnover through the sump. To note, I also run a WB2000 to my reactors which is ridiculous overkill right now and I have a secondary 1/2 line run wide open to the sump just so the 2000 is not choked back to the point of not really functioning. The reason I chose it was that I plan to add in an ATS system and possibly a display fuge so it will run 1 or more other tanks here in the near future.
Here is a good article on bulkhead sizes and flow for drains.
http://www.wetwebmedia.com/BulkheadFloRateArt.htm
Also you can use the RC overflow / drain calculator. These values should be the minimum you use for flow and if you go for a siphon style drain with emergency backup, then go with the minimum diameter + a gate valve to control overflow level.
http://www.reefcentral.com/calc/drain.php
This is also an outstanding post to calculate flow via a siphon.
http://www.reefcentral.com/forums/showthread.php?t=1814606
