The issue appeared to be the amount of extruded material. Changing the Slic3R Extrusion Filament Multiplier from 1.0 to 1.10 improved the print a lot, near perfect infill to perimeter and line to line. Increasing it to 1.20 now makes a very solid part.
Given the small printed part I am unsure what the effect would be on a large fill area or thick wall. May cause warping if there is too much filament being printed.