K..comparisons thin across the ground however this is one I found earlier comparing a modded stock camera, the 60d and the 60da.
http://www.astropix.com/HTML/I_ASTRO..._60D_550Da.HTM
In reading this it appears that the signal to noise ratio is better comparing the 60d and the 60da by a Reasonable amount. The guy who wrote this writes a lot about dslr imaging for astro. His basis for his numbers whilst not incredibly scientific seems reasonable.
End of the day, if Simon is happy and he is getting good results then its worth it?