I don't use an sdcard, but sideload most of my free books via Calibre/USB or via Browser. I believe I will easily never fill up the internal memory as I keep only a selection of books to be read, books in reading and some stuff for looking things up on the device. Everything else I archive on the PC and keep a small selection available via Browser in my Dropbox or on my Webserver so load as needed...
This way I don't need to worry about performance or the drawbacks of the current library and search-handling...
This kind of resembles my handling of printed books as well, I used to collect books and had a "wall full" of them - until I needed to move a few times and now I have reduced my library to about 100 essential Books, filling no more than 2 Boxes when moving. Anything I will not read again or would like to keep for looking something up I don't need to keep any more.
