I had a top line versahauler for my tundra. I used it to occasionally cary my 450 dirt bike which is around 250lbs.
Personally, I wouldn't cary more than that with it regardless of the spec. To me, there is more to it than the weight a pile of steel can cary. Securing the bike is a bit wonky and loading a 500lb bike would not be for the faint of heart.
If you have no other choice, go for it. If you do, I suggest the bed or a trailer.
Just MHO based on experience.
2007 BMW K1200R (just like my women - fast and naked)
2007 BMW K1200GT (man this bike is fast)
2009 BMW G450X (hard to beat a bike with FI)
2007 ktm 250 xc-w (light as a feather - sold)
2007 BMW r1200gs (awesome bike - but sold)
2007 BMW X-Challenge (SOLD)
2008 ktm 530 exc (SOLD)
2006 xr 650 r (SOLD)
brute force 650i (SOLD)
klr 650 (SOLD)