JJ is correct, it doesnt matter how far the spool is moved, it still should build maximum pressure, it will just take longer because of restricted flow. I am leaning toward valve problems myself, but I just skimmed thru the threadt. I am going to reread the thread before commenting further.
edit,
after further review, unless the splitter has a dump valve plumbed in the system, and it plumbed wrong. I still think it is a valve problem. As far as factory relief setting on the valve, it doesnt matter (for testing purposes), if its set at 2000 or 3000psi, You still should see the same pressure in either direction the valve lever is activated. When you change hoses at the cylinder,the problem moves with the changes. You have pressure at the pump, your cylinder will allow pressure build at each port, only thing left is the valve. With that said, if the splitter has a dump valve to allow oil to bypass the control valve on return stroke and it was plumed on the wrong line, then pressure could be flowing thru the dump valve back to tank, not allowing full pressure to build. I doubt a splitter with a 11gpm pump has a dump valve, but it is a possibility. The control valve should have a in, (pressure),out, (tank) and 4 ports, one port for each port on the two cylinders. Is anything else plumbed into the system?