There are many factors to glide ratio of an airplane, but a good indication is the aspect ratio (see more here: Wikipedia ). The aspect ratio tells us how "slim" the wing is.
The slimmer a wing, the less induced drag it will create. The downside is that the longer a wing is, the more parasitic drag it creates. So it is a tradeoff for glider designers. For low speed flight, the longer the wing, the better. But for high speed flight, a short wing is better. This can be seen in open class gliders, where the trend the past 30 years have been to create greater wingspan gliders. The last few years, this has changed, and glider manufacturers are now producing new generations of open class gliders that have shorter wingspans than their predecessors(in the case of the new Quintus vs its predecessor, the Nimbus 4: 23m vs 26,5m)
In gliders, a high aspect ratio in nearly all cases (there are exceptions) translate to a high glide ratio compared to low aspect ratio gliders. Examples of gliders from the same generation (or close to the same) and their aspect ratios and (theoretical) glide ratios:
(AR: aspect ratio; GD: glide ratio)
LS4: AR: 21,7; GR: 40
Ventus 1: AR: 23,7; GR:44
Discus: AR 21,3; GR 42
Nimbus 3: AR 35,9; GR 58
ASH25: AR 39,8; GR 60
Now an extreme example that has been produced in very few numbers; the ETA has an aspect ratio of 51,3 and a glide ratio of 72.
Now, enough about gliders! If we take airliners, then the Gimli Glider (Boeing 767) had a reported glide ratio of 12:1 in an emergency situation.
A 767-200 has an aspect ratio of 8~, along with a glide ratio of (according to Wikipedia) 12:1.
An Airbus A340-300 has an aspect ratio of 10~, and a 737-NG with winglets has an aspect ratio also of 10~. On the other hand, a 787 has an aspect ratio of 11~.
But without factory numbers of any of the airliners, it would be hard to say. But in any case, going by the aspect ratio, the 787 should have a better glide ratio, compared to the other airliners i mentioned.
With regards to the 787 wing features, then the leading edge sweepback is very popular among glider manufacturers today. There is also a noticable increased dihedral(wing bending upwards) further out the wing, this is also seen on gliders.
It is hard to suggest if these factors will increase the glide ratio of the 787 compared to other planes, as the wing is certainly designed for optimal performance at cruise speed, and the speed for best glide ratio is likely alot lower.
The only conclusion i would guess at, is that because of an aspect ratio that is about 10% higher than comparable airliners, then the 787 gliders better (but i have no idea of how much). If i would have to ballpark it, i would guess somewhere around 16-18 in glide-ratio.