What's the difference between MySQL 5.7 and MySQL 8?
.TRG, .FRM, .TRN and .PAR files no longer exist in MySQL 8 as Oracle changed the way they handle physical files. Within MySQL 8.0, the data dictionary has changed to include a transactional data dictionary that can store info about the database objects. There are some new features such as CTE (Common Table Expressions), regexp, windows functions, and even Invisible Indexes. In regards to regex (regular expressions), there is now Unicode support. There are some new security enhancements such as caching_sha2_password replacing mysql_native_password as the default authentication method.
MySQL 8.0 should perform better and shows to be more efficient during benchmarking. It performs very well for read/write Workload versus MySQL 5.7. There's no reason not to use MySQL 8.0 if you're able to upgrade.
Please see the following documentation for an in-depth look into MySQL 8.0.