I repeat, if you want a completely new and pleasant PES2013 experience, delete your OPTION.bin (or move it to a backup folder) then start the game, pick your settings and load or start a new ML. Get to the first match, then ALT-TAB out and put write protection on your new OPTION.bin. (the game creates a fresh one on startup if there isn't one present)
Now, the friendly AI won't deteriorate anymore, and opponent AI won't cheat you anymore. THIS IS THE CURE FOR PES 2013! (and most likely any other PES using this engine). Simple, yet effective.
I do not recommend Tenacity tweak (99 or whatever) or any other global ability tweak in combination with this mod (for lack of a better word) as the AI will work like dogs with default settings. Using this mod WILL improve movement, tactics and strategy will have much more impact and you will basically not recognize the game in many ways. One can wonder what the hell Konami were thinking when they hid this gem of a game behind all that poor programming called TeamVision (and any other crap they save to the option file) that completely destroys the game.
I assume further testing will be done extensively by Matt10, and he will most likely come up with any global improvements if he find the need for it. I use none at the moment, and I am plenty happy. The in-game speed settings doesn't even seem to have adverse effects on the game any more.
Have fun.
PS. I don't know exactly how this game works, but I do know that the engine collects data of your (BAL/ML) progress, opponents and stores it in the OPTION.bin file. Then before a match, it calculates this and that based on previous matches, what kind of game you're entering, who the opponent is, your teams stature and so on. So, if you play a good team in ML, and enter the CL against any opponent and have had reasonable success in the game so far, you will get cheated like a bitch because the OPTION.bin told the AI to do so. Now if you have a new OPTION.bin, the file will tell the AI that you haven't really played that much before and therefore want to give you a difficult but pleasant experience. OPTIOIN.bin is like a drug dealer, it treats you nice in the beginning making it all seem fun but will start to cheat you badly once you're hooked. So, I say LET'S DENY IT THAT PLEASURE! (right click the fucker, and apply "read-only" on the file and you're safe!)