frogcw321 I shoot league soccer with the AF-S 200-500mm f/5.6E ED VR on a D610 with excellent results... That said it's certainly not at the same level as my AF-2 200-400mm f/4G IF ED VR... especially the tripod collar which works but even Nikon states in their user manual to lock it down while shooting... with the 200-400mm the collar can be set with a light drag or free wheeling... which is nice feature for following fast action... The 200-500mm will jump out of its track and jam if allowed to free wheel... gota keep it lock down once you're setup... experience is a brutal teacher here...
As for the D300? Much depends on what you will be shooting... I could not use the AF-S 200-500mm f/5.6E ED VR on my APS-C (crop) D7200 for soccer... A 750mm angle of view made it nearly impossible to follow the action thus it only made sense to use this glass on full frame for this scenario...
btw if you are shooting static subjects the 200-500mm's VR is absolutely amazing! Actually believe it's slightly better than the implementation on the 200-400 f/4... But since I only use these long lenses for action sports I never activate VR... Even the 200-500mm at f/5.6 in bright daylight allows for at least 1/1600 or better at ISO 200 (which is the basement for the D300)... I still have my D300 (love it since it can pump out 8 fps with the Nikon MB-D10 Battery Grip attached) However you'll need to drop your bit depth to 12 rather than 14 to get that amazing frame rate... only the D500 can match it for Continuous High Speed release mode in Nikon's DX lineup and only the top Pro Level bodies actually can do better i.e. D3; D3s; D4; D4s and D5. That D300 is still remains an amazing sports/wildlife body.
Hope this helps frogcw321... Or is at least food for thought... I wish you well...