I was under the impression the primary benefits of the process design were containing failure and jailing vulnerabilities to avoid them from accessing your other tabs. Why not do one process per iframe/origin?
More processes do have benefits, but they increase memory usage, as measured in that link.
AIUI It's about separating all untrusted web content from everything else on your computer. As long as you have _one_ content process (and one parent/browser process), you get the vast majority of security benefits (by limiting the capabilities of that child content process).
The same origin policy already works fine, when talking content-to-content.