Hi Stu,
I have both a D300 and a Sigma 500/4.5 (not the current version) and it works fine wide open, though I usually stop it down a little to get some DOF.
Are Nikon really saying that their AFS300/4 will not work on the D300
That's a highly regarded lens and it's a pro spec small sensor body. There's absolutly no reason why that shouldn't work.
I also have the AFS300/2.8 and that's a stunning lens even wide open with a 1.4 convertor (and a D300).
Try posting the same question on Naturescapes.net, then go beat Nikon up some more.
BTW what part of the country are you?