Couple of factors that really drive the point of diminishing returns:
1. How much range does the pack have (meaning usable/realistic range) under sub-optimum conditions (temps near freezing, driving around 70-75mph?
2. How fast can the pack be recharged?
3. What is the distance between recharging locations/how ubiquitous are they?
Put this in ICE terms...there are plenty of very popular vehicles (think Honda Odyssey minivans which are sold by the millions) which only have a range of around 250 miles. However, they can filled in under 10 minutes at any gas station. Obviously in a Tesla, the need to use a supercharger limits locations and it isn't as fast as a gasoline fill-up right now, although with the bigger battery packs, we are approaching the ability to charge for around 15 minutes at every supercharger on a route and still complete the route or charge longer and skip a supercharger (note there are some places out west where spacing is such that doesn't work).
Elon has previously said that, based on data they are seeing, the practical pack size is around 100 with a range of about 300 miles and that the likely improvements they will focus on will be the charging rate and getting more superchargers out there (i.e., points 2 and 3 above).
If the new focus is really going to be 2 and 3, the difference between a current 100D and a 90D is pretty meaningless, since they are both only impacted by 3...2 is unchangeable given current battery chemisty (yes, I know that the 100D pack is larger and thus can charge at a slightly higher rate for a given amount of charge, but the difference is minimal).
We know that the 2170 batteries are coming and going to the model 3 cars first. We expect those batteries will ultimately migrate to the S and X models in 3-4 years once the Panasonic contract is met (although we are speculating on this). We have also heard indications that there will be even faster superchargers coming out "soon" (whatever that means). We don't know if existing cars and 18650 batterypacks will be able to take full advantage of those new faster superchargers or if that will be limited to cars that are 2170-based and/or have a different internal configuration in the car, so everything there is speculation.
I agree that there are people for whom the additional capacity between a 90 and a 100 makes a difference in some edge cases (rural areas or areas away from the major interstate highways where the superchargers are primarily located. I believe that for most people, those edge cases don't apply or apply pretty rarely (a couple of trips per year), but that doesn't mean that the edge cases aren't real. Unfortunately, until superchargers (or some other high-speed chargers) are darn near as ubiquitous as gas stations, the edge cases are still there and real.
Given everything we have seen, I suspect (and this is just speculation) that what we will see in the next 24 months is:
1. the model 3s will come out with 2170 batteries. The battery chemistry may allow charging at a faster rate than the 18650 packs, but I'd rate that at 50/50 because their supposed big advantage is energy density, not charge rate.
2. the model 3s will have much smaller physical packs than the S and X model (the car is much smaller), so will hold less cells. As a result, the higher energy density will result in a car with range between 200 and 300 miles depending on what variant a person buys.
3. the model S and X will stay at 100 as the largest battery pack for the next 24 months until a redesigned unit comes out in 2019-2020 which is based on the same physical size pack, but the 2170 batteries. The increased energy density may give us up to a 130 or so (assuming a 30% increase in energy density which is a LOT).
4. Some higher speed supercharger will start being rolled out which is compatible with all current supercharger enabled cars. It may drop charge times by 10-15%.
Again, those are all just speculation, but I think are realistic given all the factors. The conclusion -- if you want a car now and want max range, get a 100D. If you are waiting on a model 3, the range is unlikely to be greater than a 100D when the 3s finally come out. There will be a longer range S and X, but probably not until 2019.
