Boost power losses - What is "Voltage drop of MOSFET"?
Is this just the voltage drop of the FET when it's on? 


Yes, it would be the voltage drop from the drain to the source when the FET is on.


  Meaning it would just be the sum of Vout and the forward drop of the diode?


No.  When the FET is on, \$\mathrm{V_{DS}}\$ should be quite low -- in the tens or hundreds of millivolts.  That, in turn, means that the diode will be reverse-biased.  It is when the FET is off that the inductor current will flow through the diode.