Payload Range Diagram

After my last post a debate started about the range of the B737MAX-8 and -9, precisely whether these ranges can only be achieved with an additional fuel tank. It seems like (and Scott Hamilton pointed us to that) that Boeing already said that the MAX-9 range of 3595nm is reached with an additional fuel tank. Boeing did not say anything about the MAX-8 fuel tank configuration to reach 3640nm of range, but it seems logical that this will also be only achievable with the additional fuel tank.
A further hint to that is the difference between the different OEW and MTOW gains, relative to the NG.
OEW is said to be 5500lbs more but MTOW is raised by 7000lbs. These 1500lbs difference could mean 1500lbs more payload for a given range or 1500lbs more fuel (assumed there is a tank that can hold the fuel). With an estimated nautical air mile performance of 0.08nm/lbs fuel that would mean an additional 120nm of range. So the additional range due to the better aerodynamics of the aircraft and the better SFC of the engine would be 540nm-120nm = 420nm. This represents about 13.6% of today's B737-800NG range or 13.75% of today's B737-900ER's range. Boeing seems not to be overpessimistic at this point about the performance improvements, I would say...
As well, 3760nm of range at least for the A321neo seems a little bit odd, unless there is another fuel tank installed in the aicraft. A full somparison of range (estimations) and more can be found at Leeham News. They are not quite macthing the ranges given by Boeing for the MAX.

To better understand what options airframers have to improve the payload-range performance of an aircraft, here are some basics.
Let's start with a nominal aircraft. You can fill up the tanks and go without any payload and fly a certain distance (A). If you add payload you won't go as long as before as you need more thrust to lift the aircraft. At one point (B) the Maximum Take Off Weight (MTOW) is reached. You can only add more payload if you drop fuel. This will lower your range further, as you start with less fuel in the first place and the added payload - other than the fuel - does not disappear over time (well, unless you are flying paratroopers...).
Note that netween points B and C you always take off with MTOW. Point C is where you have reached the maximum payload - you cannot load more passengers or cargo into the aircraft.

Now what happens if you add a fuel tank? The tank itself has a weight. So the payload could be a little bit less (depending on where the tank is located). So until you reach MTOW at the new point B you can fly longer. This option does not make sense for you (as an airline) if you always fly with a lot of payload, as you cannot make use of the extra tank.

If you raise the MTOW without adding a fuel tank you can add more payload until you reach the new MTOW - or you can fill up more fuel and fly longer distances with high payloads (re the first comment: given that you also raise MZFW). This option has no benefit for short range missions.

Only if you simultaneously raise the MTOW and add an additional fuel tank you have more range for every mission.

Lastly, a better engine SFC leads to more range for every given payload.

Now everyone can build it's own "Dreamliner". Take a given (existing) aircaft, add MTOW, fuel tanks and/or new engines and see what happens...I wait for some clarifications on how Airbus and Boeing want to reach the advertised ranges of the neo and MAX aircraft.