Ralph addressed this very specifically in “The Maltby Playability Factor, Understanding Golf Club Dynamics” book on pages 91 thru 93. This was the first book published on the MPF for irons back in 2004. He actually posted the measurements for 9 different sets of irons that represented a range of design and playability for that time. Ralph states ” It can be seen that the Playability category will generally remain the same throughout the set. Generally, but not always, the only irons that tend to change to lower categories are the 8 and 9 irons. This occurs in some cases because the loft angle of the short irons coupled with the higher Basic Vertical Center of Gravity location and the Rearward Center of Gravity location can sometimes cause the Actual Vertical Center of Gravity to be in front of the face plane and not behind it, which is usually the normal case.” Ralph goes on to explain further and there is too much copy for me to post here. To answer your question, you could design long irons and short irons to have even more than the normal variation, but cosmetic transition form the short to mid to long irons might not be a “normal” progression. Progressive weighting in irons has been going on for quite a while, vertically. Horizontally, not so much. Progressive total head weighting has been the norm for decades. I think if you could read this section of the book it would help to explain it further. A key point Ralph makes is in describing the purpose of the MPF. “It is to put an iron design into a general category to help tell a player how easy or difficult a particular design is to hit. A few MPF points either way or even a a category change up or down by a few points makes little or know difference. However, the start of a category (say 551 points for the Game Improvement category), to the end of the next category (say 850 in the Super Game Improvement category) is a BIG jump in playability.” We believe that when you start comparing irons that are 150 to 200+ points separated, there can be noticeable playability differences. For some players it may take more separation to notice. Others will swear they can tell the difference 50 points. Just know that the more the separation, the more noticeable difference there will be for a wider array of players.
Hope this helps.