Considering a move to SI would love some advice.

[Nox]

You could buy:
XSI + Sub for 2015
or
XSI + (let's say) Vray

So, great render engine or couple of bells and whistles in 2015? You decide.

One of my calculations was, if Autodesk were crazy and EOLed SI on the day I bought it would it really matter? My current 3D applications, Modo and C4D, are years behind SI anyway so, no, it wouldn't matter to me.

Thats it.