Tsearch & functional indexes

In the tsearch2 documentation I see a lot of examples where you
add a column of type tsvector to your table and then indexing
that column.

Instead of adding the extra column, would it be possible to
just make a functional index something like this:

   create index foo__tsearch on foo using gist (to_tsvector('simple'::text, text))

A brief informal experiment I tried suggests that
the create index command works; but that queries
using this approach were slower than having the
column around.  I would have expected it to be faster,
since the disk space used should have been smaller.
Perhaps something about table statistics that I'm
not considering?