should I be using spray or play for highly scalable and efficient rest json processing?
In my opinion, the complexity of managing two frameworks is going to far outweigh the slight performance improvement you might get from using spray.
It also depends on what features you need. Play is a full stack framework, which is why in simple scenarios, it's slower. If you implement some of the features built into Play on top of spray, you will probably end up worse off (performance, security, and capability wise) than if you just used Play.
Filter that benchmark list down to only full stack frameworks and you'll see that play is actually quite high on the list. Comparing full stack frameworks with micro frameworks is really quite misleading.