I tried to compare the item flow speed on packed ice and blue ice when being transported by a water flow. I noticed that items always arrive at the sime time (with some minor variations) when being shot out from two droppers simultanously, sliding along identical water conveyors with packed ice/blue ice on the bottom and arriving at a golden pressure plate detector, hooked to a lamp (the attached screenshot shows the contraption).

Items are clearly faster on blue ice than on packed ice as long as they're in the dry, but I would expect them to show this behavior in flowing water too.