Newbie and string of 300 WS2811s
I don't think you'll have any problems. The LED's have a data rate of 800kbps, so you can theoretically push out 110 frames per second if your Arduino does nothing but transmit data. If you keep the routine that updates the colors fairly efficient then you'll have nothing to worry about.

Note that the array storing the colors would take up 900 bytes of memory, leaving you with only 1100 bytes for everything else your program would need to store (which should be fine).