I have owned that lens and it is very nice. It works really well with the 1.4 teleconverter, making it a 420 5.6. As you know it is fairly compact but isn't something I would want to carry while hunting. I think most will tell you the more reach you have the better, but I think you will find larger lenses a burden to carrying and take care of while hunting. I have found 300mm's and a 1.4 teleconverter to do pretty well overall, a good compromise between focal length, lens speed and price.
A zoom lens that may be worth checking is the new version of the Tokina 80-400 5.6. It is probably the most compact lens for it's focal length and unlike some other 80-400's, or 100-400's, it has internal focusing which keeps it comapact even at 400mm. Most of the reviews I have read said the build quality is great and the optical quality very good. Most said for the price(@$650 at B&H) it was well worth the money. Probably not as highly rated as a Canon 300mm or 100-400mm, but it cost less than half of those lenses. Good luck but be careful, photography can be very addictive---the season never closes and it provides even more excuses to get outside.