I want to be really clear with you with this question. Are you saying that Tegra 4 will be ready for wide distribution by the time Windows 8 comes out? I am not talking about a reference CPU but full scale production without yield issues at 28nm.
I don't believe this to be the case. Their is going to be yield and gate leakage issues that TSMC will have to overcome first. I very much doubt that they can have all of this worked out in six months. Sorry.