Currently, it's possible to buff turrets that aren't neighbouring a Synergy Turret if you invest enough in boosting range, but it's a bit difficult to tell when that's happening.
It would be nice if there was a visual indicator by how many Synergy Turrets a given turret is being boosted (see the attached mockup). It would be even nicer if that indicator would also give pips/bars/dots of different intensity to indicate the level of the boost.