Based on your pump curve, it seems pretty straightforward that you're seeing issues with over-recirculation. At the minimum, you could be having troubles with vibration and flow irregularities.
You could also be aggravating NPSH(available) issues if the over-recirculation causes overheating of...
Gotcha - didn't realize that he had gotten feedback previously.
OP - please provide links to associated posts if they pertain to a similar system (or copy in the relevant information).
Edit: Just read your previous post - http://www.eng-tips.com/viewthread.cfm?qid=423001.
Interestingly, this one of those questions that can actually be answered with very little information. Generally, Roman, positive-displacement pumps will deliver whatever discharge pressure is required to meet their fixed flow rate.
The only way in which potentially 3" delivery pipe would...
