Cessna claimed that their 210s k,l,m and most Ns (normally aspirated) had a 735 kg useful load. In real life with must have extras added you find most 210s have a useful load of 666kg give or take 15 kg.
If "fright flight" had a payload of 480kg and was an average 210 it could have carried 266L (4.5 plus hours of fuel) and still been under gross weight.
I've been told that these aircraft are loaded up to 25% (430kg) over gross for ferry flights to Australia so I don't think weight was a factor in the poor climb rate. I do think too low a speed producing a too high an AoA thus preventing the laminar flow wing from working properly was the root cause of the PP climb rate.
Cheers RA