The whole issue on how much goes into your sump and how much returns depends entirely on where the overflows are placed! If you drill a tank near the bottom it will keep draining until it reaches the level where the wholes are drilled. In this case, without a return pump you will have a flooded sump.
If the holes are drilled properly near the top water will drain until the water level is under the drilled holes and then stop. That's probably the safest option!
Canister filters are more dangerous because if there is a leak on the canister itself the U- shaped intake pipe that goes all the way to the bottom of the tank combined with the syphon will make sure you have a flood! Been there a while ago
In this case you have to make sure that the syphon breaks should the water level fall too low. Drilling a small hole in the return pipe just below the water level will do the trick and act like a "fail safe"
Hope that helps someone
Edit: sorry for the typos, **** auto correct!