Common sense tells you to stick with a pump somewhere close to where you want to be. Obviously using a Mag 36 for a 55 gallon system is not the right application for the pump. For simplicity lets use Mag pumps as an example.
Like I said above using a Mag 36 where a Mag 9.5 is the correct choice is not advisable. But using a Mag 12 and valving it back is within reason. Same goes if you think a Mag 7 might do the trick, buy a Mag 9.5 and valve it down.
All pumps will have a certain amount of inefficiency built in, power consumption due to friction loss, turbulence within the pump volute, an impeller out of balance either dynamically or hydraulically etc. Obviously a Mag 18 is going to draw more power and create more heat than a Mag 12 pumping the same amount of water.
You have to use common sense and match the pump to the application, but slightly oversizing is forgiving and accepted practice.
This is not an endorsement for Mag pumps as I hate them myself and will not use them except to mix fresh saltwater when I need to raise the temperature a few degrees and don't want to use a heater. The use here was because they have an easy to follow numerical sizing system.
I have a closet full of them that have been pulled out of service due to heat, power and noise issues!
And tmz, the use of a tee is cut and dried. It is an old technology that was used in treatment plants and municipal water systems years and years ago when power was cheap and efficiency was in its infancy. They have since figured out power costs money and its cheaper to deliver all the water you produce rather than bleed it back to a reservoir. They now use pump control valves to add additional head much like we use a ball or gate valve to accomolish the same thing.
