This appears to be one of those "Well it works in practice but does it work in therory?" Situations. I think the issue here is the preception of a more tender helm. The boat may be turning at a rate consistent with its speed thru the water but since by your visual refernces to land you're going more slowly when heading into the current the perception is that you're turning more quickly for the speed. According to this theory then your boat's turning should appear to be more sluggish when heading downstream.