I forgot to include this comparison in this post:

https://drupal.star.bnl.gov/STAR/blog/dmawxc/update-06102019-run-9-embedding-pythia8-vs-pythia6-responses-after-all-fixes

Here are the jet-matching efficiencies for Pythia8 and for Pythia6 for all the parameters and fixes as described in the above post.  They are consistent with each other over pretty much our whole kinematic range (pTjet = 2 - 30 GeV/c).  However, we do see a some differences at very, very low pT (< 2 GeV/c).  I would anticipate we should expect some difference (since the underlying event is different between Pythia8 and Pythia6), but I'm not too sure how much.