Matt,
I'll give you the bottom line first. If you have done the Grand a couple of times you should be fine. Whether I would go or not at this level really depends on who I had in my group, particularly whether I had kids with me and how I would feel about them swimming and/or getting maytagged in very cold, very fast water.
I ran it in 2011 with a July 3 launch (between 7.5' and 8' at Corn Creek, pretty close to where it is at today) and posted a pretty detailed report here on the Buzz which you can find with a simple search if you want more info. And there are a bunch of threads here describing high water trips.
Anyway, in summary, it's not really very technical at these levels. A lot of rapids are washed out, and there are plenty of big wave trains. There are plenty of places you can flip any sized raft at this level, but the river is so wide that it's easy to avoid most of them. The one place I am most nervous about at this level is Whiplash. When we did our high water trip I got pretty sick to my stomach just looking at it (in fairness, another boat in our group had flipped at Elkhorn, just above whiplash, and I think we were still spooked from that). We scouted Whiplash for a long time and at our levels there was a sneak on the left that turned out to be not a big deal. But I think it is a high consequence rapid at high water, say above 7' on the ramp. Below 4.5' Whiplash is nothing. I don't know what happens to it between 5' and 7 '.
The other big one is Chittam, right at the end of the trip. You can either sneak it far, far right or just take a downstream reverse ferry angle to cut across a single lateral. It's not really a hard move, and you have plenty of time to set up for it, but that would be a nasty rapid to swim at high water.
I have a June 30 launch. I hope James is right, but I think he's a little optimistic. I've been watching water levels pretty closely for a few weeks. I think the river is finally on its way down, and it can drop rapidly. I would expect you should be well below 7' by the 28th but how much is hard to say.
The Main is a great river; if I end up launching on the 30th it will be my 5th trip. Assuming weather is good (and the long-range forecast looks good) you should have an amazing trip. But Whiplash might well make you nervous at this level. Chittam will be huge, but if you've done the Grand twice you'll have seen comparable water.
I really struggled with a go/no-go decision a few years back at this level, and I am sort of struggling again this time, but that's more a function of a couple of passengers I'll have with me. But I'm sympathetic to your quandary. If you want feel free to pm me and we can set up a phone call and I'll be happy to tell you what I remember.