check the vendor for the displacement sizes.
If it is a barnes, the small section (low flow high pressure) is exactly the same between the two. Only the larger section is different.
therefore, since the flow is the same, the engine will run either one to the same high pressure setting, 2500 or 2700 or whatever, until it bogs the engine.
when in high flow mode, it will bog the engine at a lower pressure. That is easy, you just reduce the unloading setting on the pump. So it just shifts to low flow a bit earlier. Maybe 50 or 100 psi lower, not much.
Check the vendor numbers, but I would go the 13 gpm. I repowered our 4 inch 5.5 hp 11 gpm to 13 pump because I needed an 11 for another project. Rather than buy a new 11, I bought a 13 and moved them around. Works great but barelynoticeable. helps on return cycle especially when waiting to rotate the wood and go through for the second pass.
Be aware though it will only increase the speed about 10% or so.
kcj