From their website and from the looks of the mouthpiece the FatBoy blank seems to be very similar in style to the Megatone
Snippet from the website:
If you have had success with heavier blanks in the past, then a GR Fatboy Blank may be for you.* This blank offers all of the benefits of the GR Classic and Studio Blanks with a very solid core of sound.* Some players may find slower flexibilities and a "dead" feel on the chops, however, those who have enjoyed heavier blanks in the past really dig the Fatboy!
I have a great article written by a player in U.S. Army Band, Pershing's Own about more mass mouthpieces. (see:
http://www.ibowtie.com/mpcmass.html ). I am not sure about "dead" or "slower flexibilities." I think a more mass mouthpiece projects more -- probably better for ensemble playing than solo or lead work. But many different mouthpieces are made because players differ and have different needs.
I think if you liked the Megatone and wish to try a GR, why not try a FatBoy? It seems to make sense to me.
Jim