The IQ from the 300 ver I is outstanding from what I have seen, I'm not sure if ver II improves much in that department.
From what I've seen and Bryan Carnathan's website, no the Version II does not offer an enchanced IQ; the selling point was updated IS motor and less weight. Here's what I did. I saw the price of the new one, $7299 at B&H. I then found a version I that was actually in great shape, including the lens mount and the glass was clean, and bought that for $4500. I don't mind the weight since I'm at a fixed sporting event site when using it. So I was able to save $2800 and am getting the exact same IQ.
For the 400 I did the exact same thing. Found a fantastic version I copy for $5999 vs. the newest price around $10,900 or some even for $11,400. Again, same IQ, more weight, $5k cheaper.
Since I saved $7800, that I'd say was a great deal. Not that spending that kind of money is a great deal, I'd just say relatively speaking $9500 is cheaper than $20,000.
Will I ever spend that much at once on lenses again? NO.