Ed,
Rgarding erformance, if your backhoe has a stated GPM requirement, and your tractor hydraulics (after you take into account reduced flow after the 3ph, power beyond, valve body restrictions, etc), then your performance should meet or exceed the specs of the hoe.
If your GPM is less than the specs on the hoe, the speed (performance) of cyclinder travel will be slower, but no less powerful (breakout forces), given that your PSI is adequate. In this instance, adding the PTO pump will mprove speed, but only if the PTO pump meets or exceed the GPM requirement of the hoe.
One possibility is that you increase the speed of the cylinders to the point that you cannot effectively control the hoe (swing past your intended object, jerking up from the ground when the rock frees up, sending a shower of dirt all around). This can really be an adverse factor on performance, though you could put restrictors in the valves to slow the fluid down (back to a lower GPM, eliminating the "benefit" of the PTO pump).
My advice is, if the hoe is working reasonably well, "improving" it may not actually be better. If the hoe is oversized for the hydraulic pump, it may be oversized for the tractor as well, but an external pump could generate the flow you need. The real benefit of an external pump is to separate the flow of fluid from the tractor, either to prevent getting stranded, or more importantly, to eliminate the possibility of contaminating the tractor hydraluics when dirt from the hole is sucked into the ruptured line. We all know the worst enemy of long life to hydraulics is dirt!!
