This threat contains a few good diagrams of various pumps:
http://mbworld.org/forums/m275-v12-b...ion-pumps.html
The drawn CC circuit resistance curve in there fits your system well as it shows about 0.5 bar at 25 l/min.
You may try to bypass the pre rad with straight pipe to judge the max potential of a replacement.
If you would draw that exponential curve in the CD data sheet for your system, so through the pump curve at 25/l min and extrapolate, it fits well to more than 1 bar of pressure for 36 l/min as provided by two pumps in series.