What is the importance of dc-dc booster efficiency?
if booster can handle the load (Vout and Iout) according to design given by TI what is the disadvantage of having 40% efficiency over 70% or 85% efficiency design?


It'll get hot, and waste heat.


  will booster waste energy as heat?


Yes.  You can figure this out using the first law of thermodynamics: you have a system into which electrical energy is going, and out of which is coming electrical energy.  There's 2.5 times as much electrical energy going in as coming out -- that means the other 1.5 times is coming out as heat, because there's really no other significant energy-shedding mechanism.


  is the booster unstable?


Not necessarily.  Efficiency and stability aren't necessarily related.


  will bolster fail at some time (within calculated load range can there be a voltage drop after some time?)


That depends.  For every watt you put in, you'll get out 400mW, and 600mW will get burnt up as heat.  If your booster's thermal design (meaning -- if your circuit isn't big enough) isn't up to shedding that heat, it'll get too hot and burn up.