Well I woudldn't have mind more a more challenging Stealth aspect - even without powers. Leaning is basically as "cheated" as the "take cover in third person mode but see everything around" approach most game are using nowdays ( especially in DE:HR this annoyed me ), because as long as, standing still, you're hidden from your ennemis, you can lean and stay invisible. Which basically made Shadow Vision useless ( well I'm farstretching things here, but I think you can get my point ).
1. This thread is for giving the OP tips not your opinion on the gameplay. Go http://www.gamesas.com/topic/1417200-ai-is-horrible-from-the-stealth-standpoint-arkane-is-it-too-late-to-improve-it/ or discuss your opinion on stealth.
2. On hard and very hard Guards have noticed me while I lean.
Anyway, this is what I used on playthroughs.
Non-lethal: Blink 2, Agility 2, Darkvision 1 (or 2 if you want help with loot and walls of light), and Time bend 2.
Lethal: Blink 2, Shadow Kill 2, Time Bend 2, Vitality 1, Agility 1, and think I had Wind Blast 2.
Just go with whatever you want.