Magnetic reed switch current ratings
My question: Since the lights are 12v and NOT 20v, would stepping down
  to 12v change the ratings at all in regards to wattage?


There are are few things to think about:


The maximum current will be a fixed amount.  This is because the contacts and conductors will get hotter the more current that goes through.  In terms of current you get less power  ('wattage') for the same current as the voltage goes down, so you get fewer watts at 12V than you do at 20. 
When switches open you sometimes get sparks, this depends on 2 things: The voltage and the inductance of the load.  Sparks/arcing can damage the contacts so it's bad news. The maximum rating may well assume no inductance, and with a xenon bulb there might be quite a lot of inductance.


The easiest way to get a lot of light without a lot of current and inductance is to use LEDs