I would venture to assume that three (3!) lbs of rotating mass off the front wheel is significant enough to affect steering and can be felt by an experienced rider, riding two bikes back to back or on a road he/she is very familiar with.
The steering input points, effort and resulting lines (either under or over steer) are easily felt. I think attributing all of it to a placebo effect may be a mistake.
Riders who ride for a long time can get pretty in tune with the machines they are riding and can easily feel changes to the machine's behavior. It might be intentional mods, as well as things that go wrong, mechanical changes, strange noises, etc. Steering effort change is one of more easily felt, as far as my experience goes.
When making a change, I am always going to a road I am familiar with, riding at a similar pace and trying to feel how what I changed affected the bikes behavior. In most cases I can feel a difference. In some cases more than in others.
I ride because my wife
says it keeps me young and is better than if I get a girlfriend...