So I've been busy lately and haven't been too active on the forums or playing crawl, but wow, a 2 page thread about chei and I haven't even posted once!
Sorry if this is somewhat of an unorganized post!
I (obviously) feel that chei is fairly good so far, so I'd do gradual improvements. I think working on the stat system overall would be the best way. Make str have roughly double the impact on damage it does now, so it's worth having +15 str. Reduce or remove the deminishing returns on dexterity - it halves at 24, and halves again at 34. I don't think anyone who isn't worshipping chei is going to ever hit 34, so that's basically just a pure chei nerf. Over 24 is fairly uncommon for non-chei anyways, although possible.
And then haste needs a slight nerf - my suggestion is fairly simple, it's currently +50% speed; ie action * 2/3. I'd just change it to actions * 3/4, 25% buff to speed. That's fairly minor, instead of .66 speed, you're at .75. It can stay at level 6, with the current glow, and be fairly good. It's a level 6 spell, it should be useful. No need for drastic changes or untested systems, imho. Simple tweaks, should be fine. Of course if you want to make new, interesting content, I'm all for it, but I think the current systems could be balanced.
Of the more out there suggestions, I like the idea of taking damage later in time - taking 80 damage now vs 4 damage for 20 turns. I pretty much always run regeneration on a chei worshipper; there's no ban on necromany, and with +15 int, it's easy to cast even in medium armor. Being able to regen the damage off over a long period of time would make chei very interesting, although perhaps a bit too powerful. You'd have a hard time dying quickly, which is how I usually die
Increasing the radius of bend time to 2 would be nice as most monsters you'll end up killing in 3-4 turns. Having to spend a turn first to apply the slow tends to cancel out the benefit. Assume you just moved into melee range and you're both able to act at the same time:
You and the monster act at time 0: you bend time, they swing. at time 10 aut (aka, 1 turn) the monster swings again, and you do your first hit. You swing again at 17 aut, and 24 aut, the monster gets a third hit at 25 aut, and then you hit the third time at 31, and fourth at 38 (the monster's next attack would be 40).
You could just attack four times, kill the monster at 28 aut, and it would have swung twice, with its third attack being at 30 aut. Now of course it's worth casting when you will have to fight a monster for much longer than 4 turns, but when you're wielding a powerful 2h weapon, or using unarmed with forms, most monsters are going to die pretty fast. I don't really use bend time very much, with 3 chei wins. Being able to apply it before you are in trouble would give you a nice lead on combat, as they'd take a step at the already slowed rate, and you could start attacking immediately.