I've always used powerheads in my planted tanks. Just make sure they don't cause any surface movement. I have two maxi-jets outfitted with extra hose on the intakes and they're run under my gravel. So including my filter, I have three intakes on the right side, and three outputs on the left. Very good way to recreate a river effect... just tricky to reduce surface movement.
As far as putting the powerhead on the return line, it depends on what type of filter you get. And even then, it either does nothing or hurts your pump motor. The Eheim classic's use magnets to drive the impeller in your pump. There's only so fast they will spin. If you use a powerhead to try and make it spin faster, it might. But only as fast as the powerhead is capable of (taking into account they're not designed to have any 'head'.) And if your filters impeller is mechanically driven, you're just going to put pressure on the motor.
An analogy: You tie a rope to yourself and your friend. Your both capable of running at 12 kph. If they lead the run, will you be going 24kph or 12kph? Or they'll drag you, but not at 12kph....
You would get a greater flow in your tank by running them in series. Place the return for your filter on whatever side of your tank you choose. Using your hand, find where you can't feel the flow anymore. Put your next powerhead there. Rinse repeat.