Head gasket restrictions
Here is a pic of the head cooling circuits and your head gasket cooling circuits. The stock head gasket has restricter cooling holes in it. Only the amount of water that can flow through the small holes in the head gasket will come out your pisser. If any of these are restricted further we would expect even less flow out the pisser.
we are running the stock head and gasket with a kirker pipe and single cooling, I drilled the fitting on the bottom of the manifold bigger as well as replaced the stock heads water fitting, installed a 90 deg fitting instead and installed 3/8 cooling lines in the engine bay including manifold to bulk head and 3/8 pisser. We get more flow now, it might be a 2 or 3 inch stream at idle and 8 to 10 inch at cruise. The elbow does get warm to hot and so does the water lines.
btw with these head gaskets and single cooling among other things we don't get the flow like the big boys do, "two streams that shoot horizontally overboard 2 feet"
Ada has a oring replaceable dome head for the kaw 650 that wii give more out flow, couple that with dual cooling
the 6mm pipe from pump maybe seams right, my Friend has the ski now so can't quickly double check, I installed 3/8 cooling line on mine if that helps
normally for an accurate plug read you'd use new spark plugs