cjbecker, Regardless of the film in question, as exposure is reduced, detail is lost. The new Portra is really good but Kodak hasn't found a way to manipulate the laws of physics.
At a certain point reducing exposure will start affecting the print we expect, period.
Where that point is for each of us may vary. I'd hazard a guess that you, Andre, Athiril, and I have different photographic sensibilities and expectations.
In the absence of testing for oneself deviating from box speed is a real gamble.