One of the largest improvements I've noticed in between caching and optimizing in PHP has been turning on APC. I'm not aware of any drawbacks, if you have any let me know. I didn't do any bench marking, but it did make a significant impact on page loads to my eye, as well as the stats on the page.
While backend caching isn't what the article is about, you're absolutely right that an opcode cache such as APC significantly improves backend performance. It's one of the first things I check for when I'm optimizing a client's PHP site.
A while ago I did some benchmarks with hooking up APC and Squid proxy to the site I was working on at the time. The results were pretty amazing. I saw an increase in capability/performance of 400-700%. I'd probably choose Varnish over Squid if I had to do it again, however. The Squid configuration file is a bit of a nightmare.
