Regarding the increased range of the 787-9 over the -8, it looks like it has very little to do with the engines.
There is the hybrid laminar flow control system discussed above and some airframe weight savings (although the empty -9 is still heavier than -8), but the biggest enabler of the range increase looks to be the higher MTOW. This is made possible by increasing the gauge and strength of key structural components.
So I guess in theory the max range of the -8 would be greater but with a full load of passengers the higher MTOW of the -9 gives it it's greater range.
There is a great article about it in Flightglobal:
ANALYSIS: Why Boeing?s 787-9 is more than just a stretch - 7/10/2014 - Flight Global
And the graphic below shows the different range profile at the different MTOW options:
