My instinctive guess is that the load on the rack is faster than on your back, but control feels easier with the backpack. Also, with regard to the feel, there may be an analogy here with lighter frames "feeling" like they accelerate so much more easily than heavier ones when the actual difference is slight. But it seems more responsive, quicker.

If we can ever get a nice weekend here again and I have the energy, I think I'll do a coast-down test. I don't think the weight distribution will make much difference but the aerodynamic drag will change measurably.