At Long Lake Prov. Park (Halifax), the majority (>3/4) of Beeches had most leaves eaten away or severely damaged. Some trees were nearly bare. Unaffected trees were mostly isolated from other beeches, typically young, scattered trees.
In a forested area in Cape Breton (Lime Hills), nearly all (>95%) of the beech trees in the forests were dying or dead. Those were beech dominated hardwood forests (other dominant species were yellow birch and sugar maple), and the area was some 1 km from the next road (i.e., spread of the miner was unlikely by people).
I think the leaf miners have been around for at least 5 years. I think to assess the impact, one would have to choose a sample area, and do a bit of counting (# trees in area / # of trees affected / # of trees healthy) and follow for a few years.