Nice to know someone's still interested in this...lol.
I don't believe it's ever been officially confirmed by a dev, but this actually became even easier to prove in '14 with Player Lock.
If you still own the game, try this: Go into an exhibition game on any hitting difficulty level other than Dynamic or Legend. Choose Zone hitting and make sure the Plate Coverage Indicator is on ( i use reticle as it's the one I'm most used to judging its size). When your first hitter steps up to the plate, screenshot with the share button. Then, with the same hitter still up pause and go into the subs screen to turn player lock on for that player (highlight player, press R2 and accept the confirmation. Unpause and screenshot.
If you didn't notice immediately you should after comparing the screenshots - the PCI should have shrunk for the Player Lock shot.
Here's an example on All-Star difficulty (I realize there's not a lot of context in these pics, but this is REALLY easy for anyone to take 5 minutes and see on their own game):
Normal Exhibition
Player Lock
So why does that happen? I believe it's the same effect used in RTTS, seeing as Player Lock and RTTS both function basically the same while in-game. And the effect is that of the difficulty getting bumped up a level for a player playing in an MLB game. I even tried AA and AAA exhibition games in player lock, and the pattern holds true - larger PCI for AA and "normal" PCI for AAA.
Notice I said not to try this little test on Dynamic - that's because switching between player lock and normal has no effect. And if you apply the same logic to RTTS, it means that with Dynamic difficulty you can have an RTTS that is completely unaffected by this internal difficulty shift. That was great last year, but it's even better for '15 ASSUMING everything will still work the same - we have the ability to "lock in" a Dynamic Difficulty setting now with a slider. That means that once you find a setting that's good for you, you no longer have to worry about difficulty moving up or down AT ALL, and you can be confident that your results are purely based on skill!
I have no idea about playoffs...the reason I'm able to be so sure about the RTTS/Player Lock thing is that it's a closed environment - same pitcher, same hitter, same game conditions. Back when I tested this in '13 I took great pains to be able to create the same situation inside and out of RTTS - meaning I edited a pitcher's and hitter's ratings to be identical to that of which I had in an RTTS game. Seeing the same thing happen in Player Lock mode, where there is no doubt that the game situation is the same, corroborates that test IMO.