It's passable on D700 but sharper if you decrease the aperture a bit. On D300, the IQ is even worse. I mean don't expect the kind of quality as without the TC. I like the bokeh on 105 VR but with the TC, the bokeh now doesn't look as good as before.
Just to understand clearer...are you comparing both set at f5.6, with TC the Bokeh is much worst than without.
As 2x TC will give you f5.6 at the widest. If possible, can you post the two pix for us to see. Thank you.