The nearest I can find is http://www.uspirg.org/reports/usp/representation-without-tax... citing 18% paid on expected taxes of 35%. Which makes an extra 220 bn - almost double.
Given that the avg net margin is 6.3% (see below) doubling the effective tax rate would cause significant issues - but I need more data
Plus 1 for calling me out http://seekingalpha.com/article/428181-debunking-s-p-500-p