I am looking at table 310.15(B)(6) and 250mcm copper is good for 300amps.
Short, IMO that is incorrect, that table is not an ampacity chart like 310.16.
That table is a list of conductor sizes that the NEC will allow for certain size services. They have not 'changed' the ampacity of the conductors, they are simply relying on the knowledge that a service sized per the NEC will not really be loaded to that level. Much like how the power company sizes their conductors.
That said IMO if they wanted that table to carry up to 600 amps it would.
I agree with John (Hotline) that there is no reference to using that table in parallel.However
If I recall Don has a CMP statement more or less saying you can use T310.15(B)(6) for parallels but I don't see anything in the actual NEC to back that view up.
Another idea would be three 200 amp service panels and you could size the conductors as small as the calculated load.
Could be cheaper all around.