As a general observation of AI performance, I'm actually noticing some underperformance from the AI, especially on difficulties below Immortal/Deity. I used to play King but upgraded to Emperor earlier this year, and tried Immortal a few times to see how the jump in difficulty felt. Then, with the first AI handicap adjustment I found that the boost in their performance knocked me back down to playing at King again (in the 12-6 patch). But now with the 12-13/12-15 patch I'm seeing really, really poor AI performance at King and upped the difficulty back to Emperor. It's really hard to get an objective read on this, but my subjective feeling is that in the 12-15 patch the AI at Emperor at the beginning of the game (pre-Ren) is noticeably weaker than the old handicapped AI at King. But then once a few eras have passed the AI really starts to pick up steam, a phenomenon I never really experienced with the old handicap model. But from what I'm hearing from the Deity players, with the current patch the early-game AI handicap is still really strong, and then gets stronger and stronger as the game progresses.
However, as it stands right now, even at Emperor the starting "power" of the AI Civs feels very weak. I'm so used to having to work hard early in the game to get as much of an early advantage, and now that feels even easier to achieve. The difference is that in previous patches once you got in the lead you pretty much never lost it as the AI got less and less effective as the game went on, but currently the AI feels like it can start to make a comeback in the later Eras is you don't constantly work to kneecap your strongest AI opponents. That's a really good thing, in my opinion, and I feel with a little more tweaking this new handicap curve could solve the issue of the game feeling like a forgone conclusion once you reach the Ren/Industrial Eras. From the perspective of a King/Emperor player, I feel the AI is still a little too weak in the Ancient-Medieval Eras. I'm not a mathematician, but if the current handicap curve could be adjusted to give them slightly more yields early on while not boosting them anymore than they currently get in the Ren+ Eras I think it would be perfect.
EDIT: I just wanted to clarify that I meant the early game AI bonuses need upward adjustment for difficulty levels below Immortal. Every tick down in difficulty level seems to have a huge effect on early game AI performance. In the 12-15 patch the early-game difference between King and Emperor feels huge. Maybe that's by design, but I feel like maybe the AI sees too much of a drop in early game power for each tick down the difficulty scale from Immortal to Emperor to King to Prince, etc.