+6 or higher for one single attack is fine.
+6 or higher for multiple attacks for a single turn is extremely questionable.
I disagree. I think high bonuses on a "nova round" or for the entire party are an assumed part of 4e. Without them you can't take the monsters down in a reasonable amount of time, and you get grind. Yes, the "downside" of this is that not every encounter is really challenging in and of itself. These bonuses come from dailies, and you most likely have to stack several of them to get up to +6 or better. Deciding where and when to use your resources is a big part of 4e.
+6 or higher for all attacks until the end of the encounter is broken.
Again, I think that depends on where you're getting these bonuses from. And honestly I don't know where you're getting those bonuses from, on an EoE basis. Most EoE attack bonuses that I've seen (even at Epic) cap at about +2, and they're mostly power bonuses. +2 more from CA, +1 from Onslaught Arrows if you have a ranged weapon user... OK, so yeah, you can do it with a daily, coming up with an extra +1 shouldn't be that hard.
So at level 24, by your math you have an optimized base attack bonus of ~29, +6 = +35, vs. an average 38 AC on a level 24 monster? Is that right? This assumes that a Leader in the party uses a daily with an EoE attack bonus - you maybe have 2 of those all day. And it assumes that you've got combat advantage, which yes you should probably be getting more often than not. And it assumes that you can rack up another net +2 in bonuses, and that you aren't taking any penalties (or losing your attack due to stun / daze / immobilize / push + knock prone / etc.). And it assumes that you aren't fighting a higher-level monster, or a Solo / Elite with higher defenses...
I think that 4e expect a certain level of optimization, maybe even a certain level of "cheese" (at least from some points of view). It seems to me that at least some of the people who I see complaining about grind are also complaining about certain powers / items / etc. being too powerful. As I said before playing 4e is about using your resources effectively. The only reason I find the Iron Armbands off-putting is that they don't actually seem to use up resources in a meaningful way. Same for the Onslaught Arrows: at 25 gp apiece their cost isn't even worth accounting for at epic levels compared to the cost of normal arrows, and that +1 item bonus to attack is still pretty sweet, unless you're getting an item bonus from somewhere else. There are going to be instances where you'd rather have elemental ammo of some type, but there's no real reason not to bring both...
But the majority of bonuses, and all of the more significant ones, come from resources that do matter. The level 27 Onslaught Arrow is pretty pointless, because even if you're making due with a +4 or +5 bow by that point, it's not worth a 260,000% price increase. Even at Epic levels you only have 4 class daily attack powers, and only 2 of them will ever actually be Epic level. You need to hit your targets, you need to hit them consistently despite all the auras, conditions, etc. that Epic level baddies can put out, and you need to deal a significant amount of damage.
Even at low levels I'm finding that encounters that aren't significantly overpowered can turn into a cakewalk. Sometimes it's a matter of timing and maneuvering, good play and/or luck. That's fine - it happens and it certainly swings both ways. At some point the party is going to learn about the monsters' push when it knocks the Leader off of a ledge and out of healing range, and then things are going to get really interesting... Other times the players just dump a ton of hell on the monsters round one. And that's fine too, they don't have those resources later if things go wrong for them...
I'm finding that, unless you really do end up with one encounter per "day" (and the players know it up front), the average encounter can be pretty average, and if the PCs hit a patch of good luck it's liable to end up a cake-walk. I don't expect PCs to drop unconscious every fight, nor will I ever lament not killing any PCs in the course of a campaign. But every 3 or 4 encounters or so something is going to just go wrong, and things are going to get really, really hectic. The thing I think I want, if anything, is something to make gaining milestones or otherwise taking a few more risks more rewarding for the players...