RC has a head loss calculator if you really want to check the loss, but I'm thinking 500gph is a reasonable guesstimate anyhow.
Putting a ball valve on the output of a pump is totally fine. It's just additional head pressure. Interestingly most pumps seem to draw less power when throttled back this way, basically it's the speed of the water that seems to be directly related to consumption, not how high the water is being lifted. I'm not sure if this is true for all pumps but most pump curves that I've looked at seem to indicate this.
If the issue is noise though, I'm curious how you have the overflow setup (Durso, Stockman, etc. ?) and how the pipe enters into the sump. If the pipe comes straight down, you get this splashing effect. Sometimes lowering the pipe so that it exits just under the water surface can help. One thing I've found that makes a big difference is if you can have the pipe come in at an angle, then the water "slides" into the sump as opposed to "falls/splashes" into the sump.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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