So I actually went through with it and dyed my hair. It went from looking like this (above) to this (below).
Oh my. This is exactly one day after I henna-ed it, so it has mellowed out a good bit since then. It's now a pretty solid red. I missed a few spots, since I put it on dry hair, so I might go back when I start seeing roots and redo it a touch. Anyway, it's a big shock and rather hard to get used to, but I really like it.