Well, you can be in the Room of Doom safely below 4-5K, but somewhere above there it starts getting sticky, depending on your skill and energy level. In the teens there is an eddy fence surging 2-3 feet high between the Room and the main current, and you would not want to be in there. There have been occasions when rafts have had to be de-rigged and hauled over the cliff on river right to get them out of the Room. Not saying kayaks couldn't get out at higher levels, but it certainly would be an adventure to try....
And Caspian's comment about the violent eddy/seam on the left of the Rock of Shock in the mid-teens is right on. The water surges up on the Rock 3-5 feet high, with a good portion of the river going into the Room and a lot of the rest of the water driving straight down the sheer wall to the left of the Rock. I saw a loose oar sucked into it once, only to be seen 6-7 seconds later when it shot out of the water, about 100 feet downstream, like a sub-launched ballistic missile.
As a final note, swimming Skull and going into the Room after bouncing off Razor Rock means that it was 3k or below, which is when Razor starts to disappear. At that level you absoutely can go in and out of the Room at your leisure -- the only obstacle being the raft of floating debris that sometimes interferes with smooth paddling and rowing.
For what it's worth.