The fact that you're talking about "spec" here, in addition to your focus on individual productivity makes me wonder if what you're doing fits in with what's described in what I linked (the Agile Manifesto).
Agile isn't for every team, and you can succeed despite not communicating, but there are risks and costs involved that, if made explicit, would not likely create costs you or your team would actually want to pay.
I've worked on teams with all senior developers where the dev collaboration was mostly covered within that 20%, and minimal synchronisation was needed because we were all on the same page. I've also worked on teams with lots of junior developers, and that did require a lot more detailed planning and discussion that probably did take us close to 50% discussions overall. It was probably a necessary evil, but I wouldn't call it efficient, and it certainly isn't necessary in all cases.