I highly doubt that what you said about the sliders is true. I would assume that the engine adjusts/manages CPU and User AI separately. I would suggest posing this question in the FIFA 14 PC thread as some of the guys there may have seen something about it in the game code.
If you haven't you may want to try giving Gonira's sliders a try. His settings produce a more open style of play with more passing lanes to facilitate play through the middle, while still being challenging.
Lastly, you did the right thing in dropping to Pro. I was having the same problem until I admitted to myself that I wasn't read for WC and Legendary yet, and dropped to Pro. Spend time working on figuring out the basics: dribbling, maintaining possession, and tackling.
For dribbling use LB/L1 for throwing off a defender's marking. Also you'll find that if you approach a defender using light touches on the Left Stick he won't be as eager to rush on for a tackle. I'm able to draw about 5 fouls (great for this game) per game if they do rush in doing this.
If you're using Tactical Defending, don't press B/Circle; you don't need it. If you're trying to tackle from the front, stay in front of him, avoid running at the attacker, and let him come to you instead. You're player will put his foot out and take it away, kind of like what the CPU does. From the side, get along side him and muscle him off the ball like you would in real life.
Doing this I now win more games than I draw or lose on Legendary, while still consistently winning the possession battle 55-45 on average. If you could play well in '13, you can play well '14. It just takes a little more time to get used to it.