To draw a parallel between your geometry argument and mine (invoked
for the UV/IR radial gradient, and also for the extinction properties
of individual star formation regions) indeed sounds like a very
reasonable thing.

--I have added a reference to Roussel et al. 2005, since your
  Argument #2 for increased FUV/TIR with galacto-centric 
  distance is similar to what I employ.

If I can add a comment, I would also insist that for the global
properties you are studying, these geometry effects are not decoupled
from variations in the star formation history.
You mention this briefly in the last paragraph of section 5.4,
but I think that the paper would gain by making the argument
more prominent, i.e linking the results from Figures 15, 17 and 18
all together in the concluding section.

--Done