Well, I can't recall how much the battery pack in a Tesla costs, but its a pretty significant percentage of the car cost I think. Crudely, a 100 mile range increase would need a 33% increase (300 to 400) in battery capacity on an M3, and that's not going to be a cheap, especially when you factor in installation costs.
Plus, where would it go? It's a big jump in capacity, and I'm not sure there is much room under the car for the extra capacity, barring some major break-through in battery density/packing.
There is also a bit of diminishing returns (the rocket equation comes to mind). The battery pack is heavy; adding more batteries means more weight, which reduces the range .. so for 33% more miles you need more than 33% more battery. And the disadvantage here is that all the times you dont need the extra range (like all those short trips to the store and to/from work) you are actually paying with lower efficiency.
Would seem to me the main advantage would be easier access to less well-traveled routes (such as when doing scenic drives around the country). And perhaps finally put range anxiety to bed for potential buyers.
I'm sure we will each 400 miles range in the near future (yeah I know MS has that now), which is probably the sweet spot. But that will require tech changes which would, I would guess, preclude a retrofit to existing vehicles.