The INTAKE side of the filter opperates 100% off of gravity. Not suction not pumping pressure. But purely from gravity flow down the tube and into the filter. If you run it level you will have very very low to zero intake flow coming from the tank to keep the canister filled. While you will have less head pressure to run the pump on the outlet side, you wont be able to feed the pump due to the reduced to no flow.
In any canister filter, the weight of the column of water in the intake tube is always precisely counterbalanced by the the weight of the column of water in the return tube. For this reason, the net effect of water weight in the tubes is always precisely and exactly zero
. I.e. gravity plays no role in moving the water through the filter whatsoever.
Canister filters are not "fed by gravity", as it is often incorrectly believed. Wet-dry filters are fed by gravity, canister filters are not. This is the defining difference between open (wet-dry filters) and sealed (canister) filters. Canister filters, by design, do not require "feeding" at all. They are based on the idea of circulating continuous volume of "weightless" water. From the principle of operation point of view, canister filters do not have "intake side" or "return side": they are perfectly symmetrical in that regard. Speaking informally, both sides of canister filter always provide exactly the same "intake pressures", which meet inside the canister and cancel each other.
This is actually the fundamental principle that justifies the very existence of canister filters. Since gravity and weight of the water plays no role at all (the water in this system is effectively "weightless"), the pumps in canister filters does not have to "lift" the water. It only has to push the water through the media. Canister filters were invented specifically to take advantage of this principle.
As long as the system is filled with water and sealed, it can be placed below the tank, above the tank, next to the tank - anywhere. The filter itself and will not be able to feel any difference. The pump load will remain constant. This is just school physics.
P.S. The reason canister filters have height-difference limitation is not the pump load, but rather seal strength. The lower you place the canister - the greater the water pressure inside the canister becomes. If the canister is placed too low below the tank, the seals will begin leaking.