I'm getting tired of typing this, but swap space is not just to increase available virtual memory. If you upgrade from 8 GB to 24 GB, then with proper swap space usage, you have 16 GB that could be used for additional disk cache.

Sure, you're still better off with 24 GB overall compared to 8GB+swap whether you add swap to your 24 GB or not, but swap can still make things more better.

(That says nothing about whether the 2x rule is still useful though, I have no idea.)