Controlling 90 LEDs using one WS2811
There is an older version of the WS2811, that's the WS2801. 
This chip also has a constant voltage option, so you can control a FET with it that PWM's all your LEDs. 
Have a look at page 10 of the datasheeet!

Edit: You would need some LEDs in series (say 3) with a resistor, you can then connect 10 of these sets of "3 leds and a resistor" in parallel, to feed everything from 12 volts.

To switch high currents with the WS2811, you can connect e.g. a 10k resistor from 12V to the OUT pins and drive a MOSFET (e.g. TSM2314) with the pwm signal coming from the OUT pins.