Alpha translation worked very well because Digital cached the application's binary translation to the hard drive (and I was running 10K SCSI drives). If Rosetta 2 does this (Macs will have more storage space), then performance will be great. SSDs FTW!
Edit: Apple could add some silicon based functional unit that improves translation as well, since they design their own chips.
I would assume every JIT these days caches its output to speed things up, but Apple is going one better with Rosetta 2. It is doing static translation and converting binaries from x64 to ARMv8 at install time. For the binaries where this can be done (it won't be possible for everything) the performance should be hard to distinguish from native code.