what about multiple files and their archives in sites/default/files/[css] and /[js]?
The files are created when you enable 'Aggregate and compress CSS files.' and 'Aggregate JavaScript files.' The do have a positive effect on the performance since the browser has to do fewer http requests.

The overrides are normal as well, and they also happen when aggregation is turned off. Avoiding those means you probably have to re-write most of the css from core.