Adding a tele converter to a Sigma 150-500 seems very extreme! What kind of birds do you photograph? Are they that far?
Personally I would not add a converter to it. You would lose a lot in image quality and in comfort too if you have to focus manually. With the 1.4 converter, it will become a very slow lens at f7-8.8: you would need a very bright day in order to avoid blurry photos. A good bridge camera might do better.
With your Nikon D300 and its 1.5 magnification, that zoom will already become a 225-750mm which is not bad at all. If you need more reach, instead of wasting money on the converter I suggest getting the Nikon J1/V1 and its lens adapter. They have a 2.7 magnification so your lens would become a 405-1350mm f/5-6.3.
I think Sigma lenses usually offer a good value for money and I have no problem with mine. With Canon fit, I have the 18-200 (decent as a does it all lens when hiking) and the 17-70 (same) and with Nikon fit I have a now-old 70-200f2.8 an a 1.4 -converter. The 70-200 with the 1.4 converter becomes a f4 so while it is still reasonably fast, it is already only half as fast as f2.8.
I really liked that zoom until I got the Nikon 70-200f2.8 which was a lot better (sharper at f2.8, nicer colours, better contrast, more reliable and faster autofocus) but then it is twice the price.
I am very curious to hear your feedback about the katz-eye once you get it!