An interesting proposition, and on paper a seemingly easy distinction to make. As you suggest, the guys and girls out in the FOBs would quite rightly end up on Rate 3; given that we don't currently have different rates of OA, the fact that those in the FOBs now qualify for work of unpleasant nature allowance (or whatever it is called) is indicative of the the extra hardships faced in comparison with those in 'Slipper City' at KAF or BSN.
However, that is the only clear cut distinction you can make. If aircrew are to be on Rate 2 because they go 'outside the wire', I can only assume there will be a Rate 2a,b and c to differentiate between the threats faced by RW, tac AT and FJ / Strat AT? If I were to be ever so slightly facetious, I would hazard to suggest that once off the deck and at normal operating altitudes, boredom and DVT are the biggest threats faced by the FJ / Strat AT community in comparison with the RW and to a lesser extent Tac AT community.
And then turning to those in Slipper City who should get the least, and quite right too; after all they have all the facilities and luxuries available to them. But being the size and importance that they are, they are also targets. Hypothetically, what would you say to the parents of a Slipper City resident who has just been killed by a stray round of IDF? You would have a hard time convincing them that Slipper City was any safer than being outside the wire.
In sum, life in the sandpit is pretty crap wherever you happen to be, albeit some places are signficantly worse than others. This was the point made a couple of years back when there was a call for a separate Helmand clasp for the Afghan OSM. However, that never happened largely because it would seem to be divisive and misses the point that everyone in theatre has a job to do regardless of location. So it isn't quite so easy as to draw an arbitary line and make up policy on the back of it. I seem to recall that was how we carved up much of the Middle East in the post-WW1 era - and look where that got us!