This is just a theory, but if the biggest difference between HP-2 and HP-3 is the tolerance of the driver matching, then its feasible that one can clearly detect whether its a HP-3 or HP-2 based purely on listening for the width of the stage, and the preciseness of the imaging and placement within the soundscape.
We all know that HP-2's have a very wide headstage, with images that span along a very wide x axis - i would imagine the HP-3's would sound much narrower in this respect, that not only would the headstage be narrower, but the soundstage abilities would suffer as well.
We also know that the HP-2's are deadly accurate in being able to assess precisely how the recording engineer has the musicians mic'd in the studio or in the live performance - this i believe is one of the key attributes that lends itself to the HP-2's amazing transparency. I would also imagine that on the HP-3, it would not be so clear and the actual placement of images within the soundscape would not be clearly as articulated or as pinpoint accurate as it would be on the HP-2's due to lower driver matching.
I dont know, what do you think, just some thoughts to throw out on the table...