Which brings me to the meat of this post. ;) How strong could Tsubaki get? A lot of people seem to think that he would never be able to cut through a top Espada like Ulquiorra, but after reading the manga chapters where the Rikka fairies are introduced and thinking about it, I think he could for a couple of reasons. First of all, all of Orihime's abilities have to do with shields that can reject events. The first three fairies (Hinagiku, Baigon, and Lily) put up shields that reject outside events thus repelling (theorectically) anything from reaching whatever is shielded. The next two fairies (Shuno and Ayame) put up shields that reject internal events, whatever has happened inside what the shield is on. Lastly Tsubaki puts up both rejection shields which then reject and cut through whatever he goes through.
Of course as we have seen not all of Orihime's shields are equal in strength. Her strongest shield by far is the one that rejects internal events. This shield is so strong that it has been called a power that infringes on God's domain. It has been shown to bring people back to life from almost nothing. So the question then follows, why is this shield so much more powerful than the other two. I think it has nothing to do with the shields themselves (which all work in exactly the same way), but with Orihime. She is the type of person in whom the desire to heal people would be the strongest desire out of the three (healing, defense, and attack). Defense would be her second strongest desire and so that shield is the second strongest, but still no where near as strong as her healing shield. And of course her attack shield is the weakest of the three since that would be what she would desire to do the least.
All this to say that I think the three fairies that make up the defense shield and Tsubaki have the innate ability to be so strong that they too tread in God's territory. Meaning that there would be no attack that she couldn't defend from and no enemy she couldn't take down. Like I said before the shields all work the same, so if one has the ability to be that strong there is no reason for the other two not to be that strong as well. It's all up to Orihime. And I too await the day that no enemy can stand against her (though hopefully that enemy isn't Ulquiorra ;) but I'll take it if it is).
P.S. Of course I am propbably late to the game here and everyone else had this ephiphany years ago, but it finally all just clicked for me. Bedsides, I still see lots of posts on BA that state that Hime could never stand against anyone strong because she is not a fighter so I guess other people haven't thought this out yet either. ;)