We always have used "Aggregate CSS files" and it does help but nothing is ever perfect as X-Cart often use Google as a yardstick (!)
We've said previously that we're not big fans of Google, but their Page Speed tests do show some of these issues in more detail, albeit 'above the fold' related issues, which also differ between Desktop and Mobile devices...
On our Dev Store, we're reasonably close to solving it. Only 1 CSS issue
(Optimize CSS Delivery) left on the Desktop test but 5 CSS issues
(Optimize CSS Delivery) and 1 JS issue
(Remove render-blocking JavaScript) still left on the Mobile test
It would show some great XC5 product design awareness skills, if these scores were the other way around, as Mobile surpassed Desktop as a top purchase device some time ago (in the UK anyway).
We'll carry out the same changes to our live store so that's up to speed too, but may wait until XC 5.3.3, because we don't want to do everything twice!