I would be interested to know how long the search index rebuild takes as in my experience this takes longer than the conversion.
It may, yes, but not by much.
The convertor is in Beta5, however large chunks have been completely rewritten since then. If you're going to be testing it, I'd suggest trying a copy from a more recent CVS checkout or snapshot (both available on area51.phpbb.com). Documentation is limited during the Beta phase. Keep in mind that all parts of the Beta are for testing purposes only.