Guild Wars 2

Does it cap with luck at 300%, or is it 20% past (for a total of 320%)?

it adds 20% to the 300% for a total of 320%

I think MF is capped at 300% from essences of luck. After that, any MF buffs are applied normally. (i.e banners, food, infusions, etc.)

magic find caps at 300% from essences of luck. All other buffs stack on top. Had mine up to 522% from the birthday luck, spicy pumpkin cookie, pumpkins blessing potion, the Silverwastes mf buff, three day magic find guild banner & 15% from another guilds hero banner.