Roland,
I have used the Sigma 500mm f/4 Sport... in fact, I was on a trip with Brad Hill who famously compared the 500mm FLE to the Sigma Sport. He was using the sport for almost all of his long tele work. I was working with my 200-400VR and 300mm pf and he offered his lens to for a few hours. The Sigma is a super lens, well balance, reasonably light and made as well as any Nikon lens I have owned. In fact, my experience with his lens led to my decision to purchase the Sigma 24-35mm f/2 ART lens.
My perception of the Sigma 500mm f/4.5 matches your suspicions, and it is the reason that the lens has not been on my list. At this point, I will probably pass on the 500mm f/4 AF-s vI and wait until I can afford a 500G or the Sigma Sport. Truth be told... I want the 180-400mm f/4 w/ built in 1.4x, but that lens is not even close to what I can afford. As for the 200-500VR... well, my wife uses this lens and it is a fine optic. Were it built like my 200-400VR and had the AFS motor of a pro lens, I would consider this to be a viable option. However, I live in Minnesota where weather extremes is no cliche and I travel to some pretty weather intensive locations. As such, I require the build of a pro lens that has decent weather sealing.
cheers,
bruce