"Head" is just the distance that your pump has to pump water back up, until it reaches the tank. It's probably about 3 feet or so. Once you know this, then you can find a chart that is specific to your pump that tells you what flow you should expect at your tank. The chart accounts for the "head loss" from the additional work the pump needs to do to lift the water.
With that said though, with the 2 sump pumps and the Mag350, you probably have somewhere between 900 and 1000 gph
turnover coming into your tank. Roughly. I'd probably supplement that with a couple powerheads rather than by increasing your return pump size. That way you have more control over where you place the flow source and can tuck the powerhead behind rockwork to hide it, or place it to bounce the flow off the sides of the tank, etc. I'd also suggest a couple Koralias, at least the #2, maybe the #3.