The chap who gets 60l/min out of his EWP115 uses 1 1/4" lines (inner diameter).
But the lines are usually not the issue once inner diameters are large enough. It is the connection pieces and radiators.
The easiest way to find out where the pressure drops accords is to temporality install a manometer in a t-piece and some rubber hose and insert it at the critical locations, that is after the pump (absolute pressure the pumps pushes), before the CC (pressure drop in lines), after the CC (pressure drop in CC), before the pre rad (pressure drop in lines after CC) and after the pre rad. I expect the pre rad being the biggest restriction.
If you find it is the pre rad, consider upping to a pre rad with 32mm connections and use 32 mm to 19mm reducing joint to couple it. Also chamfer all radiuses on rad and CC inlets.
I use 19mm hoses and connections, but the ID of the pre rad barm is only 17mm!! That is way to small.
Those electric water pumps you use are not designed to provide a lot of pressure. You want to drop those pressure drops as much as possible.
Looking at this pump diagram you have a pressure loss of a tad more than 0.5 bar in your CC system.
http://daviescraig.com.au/media/786/...SpecsSheet.jpg
If you leave it as is, it still works, just not as well as it could.