The efficiency (SFC) of the LEAP engine for the B737RE (I call it the LEAP-2B as I think it needs a different, aka smaller core) very much depends on the fan diameter, as with a larger fan you can have a higher bypass ratio, enabling a smaller Fan Pressure Ratio (FPR), which translates in a higher propulsive efficiency.
For now there are four fan sizes in discussion, reportedly ranging from 63" to 70". The lower end represents
the diameter which could be used without touching the landing gear. The upper end seems to be the maximum diameter which can be used with adding length to the nose gear.
Let us compare these lower and upper end to the LEAP-1A engine, which has a fan diameter of 78".
The takeoff thrust of the LEAP-1A will be 33klbf, the same thrust as the CFM56-5B has today for the A321 application. The max takeoff thrust of the CFM56-7B(E) engine for today's B737NG is 27.4klbf. For a better analysis we would need to corresponding cruise thrust levels, which I don't have right now.
To achieve the same propulsive efficiency, the engines would need the same fan area divided by the thrust, so the engine for the B737RE would need (27.4/33)=83% of the fan area relative to the LEAP-1A. Fan area is here the effective area between the hub and the tip of the fan blades, thus dependent on the hub-to-tip ratio. Lowering the hub-to-tip ratio is a tricky thing and deemed as technology, as with a lower hub diameter the stress in the hub rises. State-of-the-art hub-to-tip ratio is around 0.28. If we start with the 78" on the LEAP-1A, we get an "ideal" fan diameter of 71" for the LEAP-2B.
Each fan diameter which is smaller means that the FPR has to be higher to keep the same thrust, leading to a lower propulsive efficiency and thus higher SFC and fuel burn.
To determine the effect of the higher FPR I used Gasturb11 (available at http://www.gasturb.de/). Of course I did not have a model for the LEAP engine, but I used a turbofan model, which should give a good answer in terms of deltas. What I found was that an increase of 0.05 in FPR results in an increase of about 0.9% in SFC.
Let us consider now the smallest discussed diameter at 65". A simple calculation shows that the fan area is 20% smaller than at 71". Thus the FPR must be 20% higher.
Todays fan pressure ratio of the V2500 is at 1.6 (http://www.iaenews.com/media/IAE_Brochure.pdf). Expect the FPR of the LEAP-1A to be in the order of 1.45. Then the FPR for a 65" LEAP-2B would be around 1.74 and the expected SFC difference would be more than 5%.
This is, of course, a very simplified approach. And to calculate fuel burn, one have to consider a few things more. The smaller engine weighs less and produces less drag, so the thrust requirement goes down and fuel burn with that.
On the other hand, if you scale the "ideal" engine down from 33klbf to 27.4klbf, you will loose some component efficiencies, as relative clearances go up.
Boeing said to decide in the next three to four weeks about the engine, then we can make a better assessment about the B737RE and how it compares to the A320neo.