Adam - I use a Shure SM58 (usually) close mic about half way across the cone and 90deg to the cabinet face. Started off removing the front panel (and hence the "ring-of-foam") but recorded one day without removing it and it made little difference, if any. Now I stick it pointing at the edge of the hole in the foam, seems to be just as it was pre-treatment. But I am guessing it would depend where you prefer your mic, and mic's placed further away might hear more difference..... dunno. Experiment. Well worth it.
I am considering a horizontal elliptical hole to spread the sound more sideways than vertically. Who knows.....