The PA strips change even with regards to function/UI in mono.
As for cpu usage in plugins, most plugins are not programmed to pointlessly process an extra unheard channel, while in mono mode, just to waste CPU, for fun. I'm sure there are some plugins out there that do just that, but it's not typical.
If you don't like having the option of using mono or stereo channels in Luna, or any other daw with the option, then perhaps they aren't a good fit for you. No big deal.