A piston pump is one of those tricky problems in CFD, requiring modelling of a moving boundary. I don't know, but doubt if the CFD solver in Solidworks is capable of that level of sophistication.
Is volumetric efficiency dependent on flow characteristics? I would think it depends on the clearance and deformation of the pump that allows for fluid to get compressed, but not ejected. Investigating the potential for cavitation - that's a flow problem.
Unless you are planning on a large amount of analysis it is cheaper to pay for a CFD analysis. It's still going to smart in the finance department. The do-it-yourself version can take a large expenditure of time to learn how to set up problems for solution and another large amount of time to gain the experience to set up problems so the answers are reliable.
If it's a long term effort then go for it, especially if you already know the problem particulars. It can pay off big time to experiment with a computer model over building and instrumenting real hardware, but it's no substitute for enough understanding of how actual hardware performs.
If loss of efficiency is due to mechanical deformation, an FEA using pressure loads on the parts can show where flow paths may open around the piston when the cylinder walls expand and piston compresses, the face of the piston warps, and so forth.
