In my example a mean to say there is a NET 4 kt difference in apparent wind speed over the surface of the water. The Gulf stream is about 2kt. So, with my numbers a 20 kt wind out of the north would be 22 kt over the surface of the water. If the wind was out of the south, then this would be 18 kt over the surface of the water. However, the difference between 22 kt and 18 kt does not account for how nasty the water would be if out of the north. Thus my problem with being stuck thinking of this as a frame of reference scenario.
I think the physics is just more complex than simply measuring the force of one type of energy against the force of another. You may just have to trust the word of people who have sailed in it.
There have been enough accounts on here, by experienced sailors who have sailed the Gulf Stream when a wind is blowing counter to it, and they have reported very rough seas. For whatever the reason, it is real.
One thing that occurs to me, is that current runs some depth down in the water: What, one fathom deep, a fathom and a half deep, two fathoms deep? I don't know, scuba divers could probably give us a better idea. Current can involve a very large volume of water to some significant depth.
Wind, on the other hand, affects mostly the surface of the water.
It seems to me like if you were to try to push your bed across the room by pushing down, and against pillows on a bedspread. The bed is not going to move, and you're going end up with a pile of pillows and bedspread at the other end of the bed.
I know that I've kayaked and canoed on the Missouri river when the Corps had the river up near the banks with a 15 knot current in the channel,when strong winds began to blow up river. Suddenly I thought I was out at sea in a storm, with waves and swells. I could see that I was going backwards. I had to paddle as hard as I could just to make headway down river.
The Corps keeps the channel 15 to 20 feet deep. That's 20 foot deep water flowing at 15 knots, and some strong winds blowing, maybe the top foot of the water surface. I can't explain all the physics of it, but I've seen it completely change the character of a river I've been floating on most of my life, into something I didn't recognize.