I agree that in my typical driving I probably could not tell the difference between M1 5W-20 and M1 5W-30.
However, I am sure that there was a change between M1 5W-30 (SL) and the new GF-4 M1 5W-30 (SM) in terms of my engines revving more freely. In the Civic and Odyssey, it was an instant surprise the moment I had the oil changed with GF-4 (SM), and the difference was big. Same tanks of gas, same weather, same roads, same driver, etc. I had the oil changed on both cars at the same time at the same dealer. The M1 lot numbers on the old SL oil being drained were the same for both cars, and the lot numbers for the new M1 SM oil were the same for both cars!
I doubt that I could do 1/4 mile runs consistently enough to prove my point - I have never done it and I can't bring myself to run a car that hard. 40-70 acceleration would be more in my league...but I'm not planning on changing back to (SL) to prove my assertion!