Bit late to see this thread and respond, sorry. The Store section of the forum doesn't get as much traffic as other parts of it do. But maybe some of this will be helpful?
1 - Premium content does not/should not really affect game performance. What those of us with most of the store get might be longer startup times, in particular if using the Launcher to start the game, and more cluttered and longer to load catalogs in Build/Buy. Relief for both can be had if one's game is running on a Solid State Drive (SSD) rather than a standard Hard Disk Drive (HDD). But if that is not feasible, then some extra patience might be required at times.
2 - I run the Steam version of TS3 as do many others here. No issues with store content. When switching over to Steam from a prior Origin install, I did have to redownload and reinstall all of my store content though because the original downloads could not cross over to the different internal game version.
3 - Now that one is difficult. I have what must be at least 75% of the stuff on your list. Some of it I use constantly, other items rarely or I just didn't care for their effects. And some I know I will want to try out but just haven't done so yet. It's really up to the individual player as to what kinds of things, including premium items, we are going to be more drawn to using. What I do like is having what feels like overwhelming choices always at my fingertips; for me, it's part of the fun of getting immersed in the game.