According to some of the guys at MFK the expected vertical flow of a 1.5" pipe that is at least 20" and has no turns or bends is around 1350gph. But I have also read at flexPVC.com that it can flow up to 2100gph.
A fast way to calculate your current overflow rate is to hold a 1 gallon container under your overflow exit and time how long it takes to fill up. Take that time and divide it into 3600 (60 seconds in a minute, 60 minutes in an hour = 3600). That will give you the GPH on your overflow as it is running at that moment. If everything is working as it should it will match your pump output (minus head loss). (Water in, water out, always staying at the same level in the DT.)
So, if it takes 5 seconds to fill a 1 gallon container it would be 3600/5 = 720gph. If it takes 8 seconds you are flowing 450gph.
To tell if you can move more volume through the overflow, AND IF YOU HAVE ROOM IN THE TANK, turn the ball valve on your overflow down and let the pump add more water to the DT than normal. Then open the ball valve all the way, wait about 2 seconds and time it again. Waiting the 2 seconds should give the overflow enough time to catch up from the ball valve being since there will be more volume at the top. This will tell you what your overflow can do beyond what your pumps are doing.
I would only say that even if your overflow can handle it, adding a lot more flow through the it may give you an increased noise issue. It all just depends on what your acceptable levels are.