Drew -
let me throw a little info out there for ya. Like the other posts mentioned, you need to determine the route in which the water is returning to the tank. If you split the flow back to the tank with two or three or more outlets, the water gph from the pump is reduced. The concept is simple, the more 90 degree angles and splits the piping does before it dumps the water out, reduces the flow of the pump. This being said, if you have a 600 gph pump at 4' head, along with piping that has two angles and splitting twice, you may just see that 600 is proboaly not a bad estimate by the time you work the pipe up to the tank. Also, if you get a pump that cannot pump the water as fast as you are draining, you can use a valve on the pipe going to the sump. This will allow you to fine tune the water coming into the sump to better match the output.
based on experiance, I assume you are looking for a definite ansewer. unfortunitly, this will exventually come down to your choice.
I may be wrong, this is what i suggest....
-dr