I'm not sure, with a bulkhead, exactly how much flow it will carry. I'm thinking you can up the flow some, but I'm not sure how much.
The inline pumps are much better although they are more expensive. You can get them down to around 400 gph
. A Blueline 30HD is rated 590 gph
@ 4' head and will cost you around $130.00. A Little Giant 2-MDQX is rated at 640 gph
@ 3' head and runs about $100.00.
As far as figuring the head loss, that is a rather complicated equation to get an accurate figure. The easiest way to do it is measure your pipe, add one foot of head for every elbow, and look up the flow chart for the pump you're looking at. A pressure rated pump will experience less of a loss than a circulation rated pump.
Another option, if you wanted more circulation and don't like the looks of the powerheads, is to set up a closed loop. This is basically an external pump with the intake and return plumbed into the tank. They work well and there is little chance of a leak.