Sure, but that doesn't mean that a crap PC will go from unplayable to great. There may be differences, but don't expect anything gigantic.
I think it's generally as good as regular Dolphin unless you use settings that don't fit a game. (Compared to Dolphin, Ishiiruka has a few more settings that can improve the performance in some games but ruin other games.) There can be exceptions, though.
I don't think I have seen anyone test it for TASing... The TASing code should be the same as regular Dolphin, so maybe TASing works the exact same, or maybe there are additional bugs that nobody has noticed yet. Do note that Ishiiruka TASes aren't accepted on this site as far as I know (though I could be wrong), so you would need to test that an Ishiiruka TAS syncs on the equivalent regular Dolphin build. I'm not sure if Ishiiruka's changes can affect the sync.
Yeah, the 32-bit version of Ishiiruka stopped being updated a while ago just like regular Dolphin. If you're looking for good performance on a 32-bit CPU, I think you'll have a hard time finding it.