Driving LEDs with Specific drivers or buck/boost regulation
I think I understand your circuit - you have a regulated 11.5V and from this you regulate to 5V for the ATmega. Your LEDs are connected in series strings with the top LED's anode connected to 11.5V. The bottom LED in the series string is shorted to ground via a 2N7000 mosfet?


  Each LED ring draws 11v @ 300mA and is PwM switched using a 2n7000
  Mosfet from the ATmega's output pins. The total supply feed is 11.5v @
  1amp, and is a buck/boost regulated supply.


Somewhere in the string of LEDs, 0.5V is "lost" and I'm presuming you have a series resistor to govern the maximum current through?

If not, this is the only thing you should consider fitting - it'll protect the LEDs: -



The graph is for a typical LED but all LEDs have the same saort of characteristic - note the graph on the left - the LED doesn't start conducting until the voltage across if (Froward voltage) reaches about 1.6V and from there up to 2V it can consume anywhere between 0mA and 50mA. The graph extends rapidly upwards and there is a danger that without some form of current limit you may draw too much current and destroy a LED.

You may get away with it given that you have many LEDs in series BUT if you have 63 and only 3 strings, you must have LEDs in parallel/series arrangements. Maybe you can post a link to the type of LED you are using because the ones in the graph I have shown have a forward resistance of about 3 ohms (constant slope part of the left graph) and if you have 6 series entities of parallel LEDs that's a total resistance of about 18 ohms which, given your circuit details is probably going to be ok but only for "normal" LEDs.