I assume you are referring to the 1999 Apex Plus and either the 2001 Apex forged or the 2004 Apex forged. As I have stated numerous times regarding the playability of iron designs, it is about the mass and dimensional characteristics. The vertical cg, the horizontal cg, the rearward cg and the moment of inertia all play a part. It does not matter what the marketing says about the design, what the perceived features are that may or may not contribute to the playability (like cavity back vs muscle back). How the mass is distributed and where the cg is determines the playability in the MPF rating system.
You can still like or love an iron no matter what the rating is. MPF is about being able to identify why some designs are really stable and forgiving on off center hits and why some are not. Why some are clunkers and why some are not. If you hit all your iron shots on the button, on the center of gravity (wherever that may be), then it doesn’t matter what the MPF is. The fact is, most of us don’t hit perfect or on the cg. And, it’s important to note that most all iron designs that have lower MPF ratings have actual cg locations that are not in the center of the face. They tend to be hosel side of center. So, unless you are good enough to identify where it is and adjust to try to hit the iron on where the cg actually is, you will not get optimum performance. I, and I think most of us, try to hit the ball in the center of the face. If the cg is there, the results are good. And finally, most misses for every type of player are on the toe side of center. If the cg is on the heel side of center, then the results on those toe miss hits will be that much worse.
Hope this helps.