[VinnyP]

For the 300 5.6 I'd only go for 1.4x that will still give you 420 at the long end a 35mm/FX equivalent of over 600mm. The Kenko Pro Plus 300DG convertors are excellent value for money. It will just about autofocus and if the light is good enough or you are happy to use high ISO then it may be able to cope. Wide angle convertors will be filter thread mounted and really are to be avoided. Sigma 10-20 or Tokina 12-24 are both excellent lenses as is the Nikon 12-24 but actually it tends to get outperformed by the Sigma and Tokina.

For Macro then you are much better off with either the Tamron or the Sigma and both are very good lenses. I have the Sigma but the Tamron has a lot of followers so I am sure you'd be happy with either.