The B787-8 has the same wing span like the B787-9: 60.1 Meters (197 feet 3 inches).
You are correct in that the as-built 789 shares the same wing dims as the 788.
As the OP stated, the original 789 concept called for an enlarged wing relative to the 788.
3 factors lead Boeing to drop the concept for a larger wing on the 789:
1) Budget overruns were already killing the 787 program. They needed to find a way to trim 789 development cost (and risk), and this was one way of doing that.
2) The final 788 wing performance was better than expectations/projections. It would perform "good enough" on the 789 for the majority of missions.
3) The proposed larger wing would have reduced cruise burn and therefore would been advantageous on the longest flights. It also would have competed more equally with today's A359 performance at range. As previously stated, due to how overweight the 787 program was compared to original expectations, the MLG didn't have additional margin to carry this heavier wing by increasing MTOW. Therefore, this heavier but better wing would have reduced available payload (and likely increased fuel burn) on short to mid range flights, which today still makes up a large percentage of the number of flights flown by the type. Boeing's decision to keep the 787 optimized in this middle range is one of the differentiator factors that have helped it against the A359. It's a great jack of all trades.
In short, Boeing determined it was not worth the additional investment (time, risk, and $$$) for improved performance at extreme end of the envelope. It was the right move, IMHO.