Does changing the similarity measure from Classic (based on TF/IDF) in Elastic 2.x to OKapi BM25 in Elastic 6.4 has any impact on overall Elastic search performance?
Doesn't look like it. Benchmarks here for a variety of queries:
https://home.apache.org/~mikemccand/lucenebench/
Shift drag to scroll graphs to the left and look for the "BI" change in the timeline