I remember counting the maximum amount of mainstat you can have to be only around 3400. I guess the tables I used for maximum affix rolls were wrong (unless barbs have some multipliers or something).
You can go over max stats on an item with legendaries. For example, Immortal King's Irons and Strongarm bracers can go over 200 strength which is the maximum for rares. This can also be done for other classes, Vile Ward Shoulders/Blackthorne's Notched Belt can go over 200 Intelligence/Dexterity.
There's a few other new ones like Ice Climbers which can roll like 260+ Str + a socket, allowing you to get like 320+ Strength on boots alone.
Oh and you can get 300 main stat from paragon levels now.