Just wanted to know if there is a performance boost if I run Steam in Offline Mode vs. just having it on normally.

Well... Steam will be active, offline or not. I think I saw threads, where some players claimed they saw somewhat increased performance. Whether this was because of Steam in offline mode or something else, I don't know.

Personally, I don't believe you'll get anything like that -but it doesn't hurt to try.

As I understood it it will make no difference if Staem is on- or offline.
But did see some mentions about a (tiny) bit of a boost by turning some in-game Staem things off.
Personally I wouldn't know cause I am always in offline mode except when there is a Skyrim update

Even when Offline Steam is still running and will still connect to the Steam servers if you have an active internet connection - so uses some of the CPU processes -- however if you have several games in your library you may see some boost in performance in Offline mode since it will not be looking for and possibly downloading game updates or locking up the computer if it is trying to reach the server and your internet connection gets interrupted etc. while you are playing - but either way it is a source of possible slow downs since it is another process running in the background possibly using CPU processes and RAM that could otherwise be being used by the game.

Generally there shouldn't be any performance boost because like anything off screen, it shouldn't be taking time CPU because it idles. The main thing with apps running in the background is memory and if you have plenty then it's not an issue. Background stuff could spike your CPU but that's more likely a bug.

Disable the community overlay crap and you're fine. No need to mess with anything else.