Hello,

Unfortunately, there are known occurrences when Plesk upgrades were unsuccessful, so I would highly recommend to backup all your data/changes before doing it. Those occurrences are rare but anyway backups could be helpful. As for the files you modified, all depends if they are the ones that being adjusted during Plesk install or not. For the first case, Plesk may try to re-write them.

I agree with you. Usually the main purpose of releasing a new version of Control Panel is to fix bugs and add new features. This shouldn't affect the website performance notably.