Both WP Super Cache and CacheEnabler have the ability to create .gzip compressed files alongside the regularly cached .htm pages. Nginx can take advantage of that (although I ran into some problems with it trying to gzip files twice) with the gzip_static option, making it look for those pre-gzipped files and serve those directly, rather than compressing them on the fly with each request.
I haven't looked into this yet myself (might be willing to give it a try), but I'm curious if this would be difficult to implement such an option in Autoptimize?