My computer isn't very strong (even playing PSX stutters sometimes if I forget to exit Chrome) so I wouldn't risk any performance for visual flair.
As for the pause menu, that would be very cool. I already use Retroarch whenever possible specifically because of its "universal" pause menu.
The best ideas for the pause menu have already been said so I'd just reiterate:
- view images like boxart and such (with zooming / panning if possible)
- view manuals
- view cheats
- view controller layout
- save / load states
- quit game / go back to big box
- current game session time
- "reset" / reload game