Jan de Mooij at Mozilla did get back to me with what needs to happen for the Baseline compiler, which as previously mentioned is the low-profile version of Ion, and that will be critical for 24 (issue 224). Baseline is a lot simpler to implement, but it does a lot less, of course. The good news is that about 2/3rds of what it requires I'd already done for Ion, leaving only some assorted leftover pieces that I need to write (I also have to get it to at least link completely, what I'm calling "Ion stage 2"; even though not everything needs to be implemented, the stubs need to be present). However, looking at how the compiler works, I'm gloomily predicting benchmark losses of around 25-30% slower. It's still faster than TraceMonkey was, but it's going to be slower than 17. I can't do much in the 22 timeframe other than get it to build completely because Baseline wasn't enabled until 23, so no testing will be possible this cycle.
In that sense, it's probably a good thing that people are only just now noticing that I dropped plugins completely in 19 (yes, it's true, for those new to this blog), so some may stick with 17 for a little longer for the plugins and as a fringe benefit will still have methodjit at near its operational peak. While I'd like to eventually get Ion up on 24 stable, it's going to be entirely dependent on how much backporting I have to do. Such is the nature of life in twilight.