Peter, it's fun you ask right now, because I happen to just be writing a SRS daemon for postfix, to be used as a tcp_table(5) canonical lookup table. It's part of my soon to be pfixtools (Postfix Tools), that you can grab on my git repositories.

I should package them as soon as the srs daemon is ready (and it's almost ready, it just needs the pidfile and fork-in-the-baground thingies).

And as a matter of a fact, yes, if you're a forwarder you have (because of things like SPF, but not only) to use a Sender Rewriting thing, and SRS is somehow elegant in the sense that if many people use it in the forwarding chain, the sending address won't bloat. But even if nobody else uses it, then well, it works, and serves its purpose really well.

As of performance: on my laptop, it's able to deal with 100k SRS rewrites in less than 0.72seconds, so it's as fast as it can be, only postfix supporting SRS natively can beat that I'd say.