I suspect the caching benefit is minimal, the vast majority of visitors won't have the fonts in cache. I'm basing my suspicion off the research done into jQuery cache hit success rates at http://www.stevesouders.com/blog/2013/03/18/http-archive-jqu...

Web performance is complicated though and all the pros and cons interact in non-intuitive ways. A comparison of self-hosted, inexpensive CDN hosted, and original Google hosted may provide some performance surprises.