Will too many overridden css rules lead to performance issues

If you care about performance then don’t use Avada theme. It is total garbage when it comes to performance. The demo has 27,000 lines/5244 Rules of CSS and 2 dozen inline style sheets.

In my experience a complex website with 30-40 different templates could be around 1000 rules and 5-6k lines of css.

No cache plugin will save the client from a .7mb css file. Most people especially theme authors in the WordPress community do not take this stuff serious. Speed is king.