You post says 20rlxt ..
Here is the info I used as a basis.
Iwaki WMD-20RLXT Pump
The RLXT type aquarium pumps are best suited for circulation only--situations where water encounters very little back pressure. The WMD pumps have American motors.
- Flow rate: 420 gph
- Max head: 14.1 feet
- Amps: .45
- Dimensions: 10.5" X 3.5" X 4.4"
- Connections: 3/4" MPT Inlet / 3/4" MPT Outlet
Regardless, If the opening coming out of the pump is a 1/2 inch then that's what the gph
rating is based on. increasing the plumbing won't increase that number. I question of we are talking about the same pump though. Your stated specs and what I found are quite different.