TL431: comparator application

Part Number:TL431

Hi team,

I have two questions:

1) I understand that when TL431 works as a comparator, the low level output voltage is about 2V, is there a guaranteed maximum value of this output voltage?

2) The following circuit is designed to meet the function that when the input voltage is under 15V, the MOS stays off, when the input voltage exceeds 15V, the MOS turns on. But the thing is when Vin is higher than 15V, the Vk voltage of TL431 is about 2V and the MOS still off. 

I'm thinking adding two diodes at the transistor emitter to ensure the transistor not open when Vk=2V, so the MOS opens. Am I right on this?

Or do you have any other recommendations on utilization of this functions?

Thanks

George