You want to try and avoid restricting the inlet of any pump, and especially aquarium powerheads whenever possible. Restricting the inlet is orders of magnitude worse for the pump and its flowrate than restricting the outlet via the same means. A restricted inlet REALLY restricts flow.
Here it is from the "horses mouth" !!! I received this in an email from Eheim !
"Thank you very much for your inquiry by email. It is possible to adjust the flow rate of the ecco-comfort filter 2234 by means off the shut off tap on the pressure side. You only have to close the shut off tap on the pressure side (NEVER at the sucktion side) till the performance you want to have is reached."