I can only repeat... no gravity involved after the filter is primed!
The longer the hoses, the more friction, the more the impeller/pump has to work to push water through. But it doesn't make the slightest difference if the canister is below, at, or above tank level.
Why do manufacturers suggest to place them below then? Because you can't get a siphon if the canister is placed above! Plus, usually ppl would want to place it into the stand anyway, so to avoid all of the "Can I place it in the stand, below the tank?" questions that's what they recommend.
Regarding the airleak, it really depends where the leak is, and again, totally independent from the placement of the canister. If the leak is before the impeller, the filter will airlock. If the leak is after the impeller, the carpet gets wet.
As long as the pump doesn't leak, and you get all of the air out, you can place it wherever. The longer the hoses, the more resistance, the less water comes out on the end.