My recommendation to not use a valve to control fine tune flow is based on my own design preference.
Can it be done in black in white terms is the same as the question can one keep anenome under PC lighting......sure it can be done, but is it the best choice?
You are correct in that adding a ball valve simulates more head pressure and Minh is even more correct that a gate valve is a better method.
A gate or a ball valve can be used to "fine tune" the flow rate and should be installed in case that the pump needs to be serviced.
The sticky point I have when technical questions are asked is I assume worst case senario as you are assuming best case senario. The worst that would occur gets by blindly following my advice is that he would get some more flow into the sump..........however, based on the facts that he presented, and that I answer questions based only on the information provided solely, and assume worst case senario, if he does follow it, there is not a safety or mechanical failure issue to be worried about.
So lets play this assumption game of design and based on the facts and description of what he said........"most of my LPS look like they are in an underwater hurricane".........even though I know the older pump though rated had probably declined in performance and he was seeing the true rating, or had a large reduction which caused the very same thing we are discussing here is what is going on. Regardless, he was not seeing the true value of his Mag and now he is seeing the full rated flow of the other for the first time.....
Based on the information he gave I am takin into consideration the worst case senario that he bought and installed a pump on a limited circuit and basing my advice on that. In other words flow rate= Area*velocity. I am thinking of the worst case senario and that he may have it ALREADY throttled back due to restrictive piping causing high velocities at the exit which would cause this effect. Giving advice to do it more could cause wear leading to shortened operating life or cavitation causing bubbles which I am sure he would be happy about.
More so, why buy a large, powerful, expensive pump only to throttle it down? It takes energy to overcome that head or "back pressure" your creating.....to overcome that head pressure, pressure must be created from energy that the motor is producing, the by product of mechanical ineficency is heat, we want that pump in it highest effecieny range all the time, that occurs when its not worked to its maximum. When throttled back and the pressure that is created it forces the pump to work harder which creates more heat and reduces the mechanical ineficiency of the pump........and we all know how much people hate pumps that heat up cuz the water sucks it right up. Not too mention heat causes wear...........
I don't know his design and to be quite honest am unsure of it and do not want to give advice as an Engineer where my advice could further worsen incorrect application of the pump......ie: too small area of pipe......he didn't say what size, it would be foolish to assume.
So, based on the WORST case scenario of him having a design that is totally flawed and answering ONLY the question at face value as it is asked, I do not make assumptions that could cause a failure by doing what I suggest.
So based on the initial information based on your advice there is a possibility based on the unknown design factors premature pump failure could be caused by simply working the pump to hard for its optimal operating range.......kinda like going 55mph in first gear in your car.......that works also right? Do you guys drive down the road in first gear at 55mph or do you use the transmission to keep the engine within its operating range?
The main fallacy in your logical reasoning is this: you are assuming the best case scenario, which in all honesty may be true and the odds in your favor..........However, adding a Tee that has a ball valve on the end of it, which I was decribing if I was not clear, will "spoil" off that excessive energy without making the pump work any harder. Furthermore, if the pump/circuit is WAY unmatched there is no worry of creating pressure outside of the optimal operating efficiency range of the pump and the flow is just diverted as if creating another outlet nozzle.
I guess it comes down to a saying that a professor told me.
"A doctor who has a 80% success rate is a miracle worker. However, an Engineer with a 80% success rate is unemployed."
In other words:
You know when people are always worried about the minor losses of the pipes down to the foot lost.........I laugh, most of the time you cant find a pump for that specific need.....say 102gph.....you may find one that does 115gph or 120gph.........the ball valve is what controls that.......if the pump is within 10-15% of the need, then a ball/gate valve is what is needed to control the flow. However, if it is higher than say 10-15% it could cause problems. But at the very least is a poor design and a waste of money. No other design data ie: how much flow he needed or what he wanted his turnover to be.
Its not my place and is rude to critize his design without being asked, therefor I will assume worst case situation so that anything I suggest will not compound the situation and "break the camels back"
I considered all of this in about five seconds and gave my response based on my design preferences and given information. Did you consider the consequences of yours based on that or are we assuming and hoping for the best case scenario?
Keep forgettin to refresh the page :/
While your talking about that point out which corals look like they may be getting too much. 
