I have now ingested pretty much all filmclips and some songs. there will be several more audioclips and animation out of Motion or AfterEffects, but I guess 5500 will be where this project will end up sizewise.
FCPX seems to handle this load of clips easily (keep in mind it uses all of my 16GB RAM right now!).
When in List mode the Event Browser is snappy. When in Icon Mode I need to wait a little while changing keywordcollection or Folder, but nothing unusual. While I stay within a folder/collection there is no delay or lag at all.
FCP7 constantly crashed on me while scrolling, when I had bins with many clips and thumbnails activated . FCPX does not mind to show dozens of clips with waveforms in the Event Browser. That is a surprise to me for a .0 release.
Then again it crashed twice today when I imported animations (ProRes422 movs). So it is not perfect yet, but it is definitely workable.
One little feature would be useful in the Eventbrowser: Now I have listview with the selceted clip showing one line, or I have icon-view where clips simply fill the lines and then drop into the next line. that´s hard to scroll through. I would like a iconview where every clip has its own line regardless of its duration. That would be a cool addition to the slider that allows to select seconds, minutes or ALL in the icon-view settings.