<li>When doubled, twice the lateral aberration, longitudinal aberration is 4 times greater rate of expansion, because the F-number is also double, longitudinal aberration is twice per depth of focus</li>
I read that as it, like the 2X, adds two stops. So a 2.8 lens -> 5.6, not 8.
you've read wrong: you loose 2 stops with the 1.4 converter, and tree stops with the 2.0 .
this means 4 stops with the 2.8
I'm sorry you're not right - you loose 1 stop with 1.4 and two stops with 2.0. Assume you have 400mm f4. It means a hole is 100mm wide (400/100). If you add 1.4 TC, you have 560mm. So it becomes 560/100 = f5.6 and f4 -> f5.6 = 1 stop. Calculate it with TC 2.0 and TC2.8 accordingly and you will find it.