I have used both Spectre and Instar. I prefer Instar for straight forward simple transforms, but Spectre can do more complex ones. Spectre has a more difficult API to wrap your head around and that is its biggest downside, IMO. You really need to dedicate time to learn it rather than try to pull it out on demand.

What's the performance vis-a-vis Specter?

I have never run a benchmark and the specter guy really cares about it so instar is probably way worse :)

I was more trying to show that the API could be better and hoping this would put pressure on core devs to do it properly.

Very nice syntax, looks great.