Pipe size doesn't have to be restricted to those on the pump outlet/inlet. You can use bushings to switch to a different size. What I'd do is use at least 3/4" or even 1" ... it does make a big difference.
You could try leaving the 1/2" bulkheads in for now but using 3/4" or 1" on the pipes, and see how it goes. The bulkheads themselves will be a slight restriction, obviously, but it might not be too bad. Failing that you may need to get the holes enlarged, but jump off that bridge only if you have to.
Basically by stepping up the pipe diameter, also going PVC with glued fittings, and maybe stepping up to a Mag7 or a Mag9 even, I'd bet you'd probably get the kind of flow you're looking for. Just my guess though ... always a little bit of trial and error at first when doing these setups.
If I had gotten a nickle for every time I redid my plumbing for every tank, I'd probably have gotten enough to pay for all the extra little PVC bits that I went through!!!!!!
