@morpie For what it's worth, I have a cocktail style cabinet and play most vertical games rotated by Retroarch (my setup has a three-sided control panel) and performance is great on those games. Actually, vertical orientation works great in other emulators as well. For example, Tempest in AdvanceMAME runs fine rotated to fill a vertical.
I still use Emulation Station as a front-end since this can be managed from the horizontal position on my system. However, I am using an LCD display upside-down (this is typically the best viewing angle for a table top cocktail design) and I need to rotate everything 180 degrees using config.txt. I don't believe my system pays much of a penalty for that, if any. But I did try running with 90 degree rotation at the system level and I can confirm that games definitely slow down.
So, from personal experience, if you can keep the operating system horizontal and let the emulators do the rotation for vertical games, you get the best performance. And if Attract Mode has the ability to rotate itself, well, that's the best vertical solution I have heard of on the Pi so far.
