Andy speaks the truth.
In lay-man's terms - having an external graphics card to power
Rome II well, would be massively compromised by the interface (the bit between your external graphics card and the laptop itself). Imagine having have a great set of slick tyres (your laptop) that connects to a 7-litre V8 (the external graphics card). Then imagine the connection interface between the two being a garden cane. Each end of the equation is more than capable in itself - it's just the bit in the middle which lets it all down.
D.