Or maybe anyone know a better place to ask this question?

Try to figure out how many of the users will actually use the system
simultaneously will be a good start...

One thing that often goes un-observed when shifting from a small-app to
big-app, is the obvious "magnification" factor. Since the application
is bigger, there are more areas of vulnerability - as in bad code,
bottlenecks, connection leaks and so on. So the first thing i would do,
is to make sure the code review "checklist" for best/worst practises is
taken care of (if you don't have one, create one - that should be the
first'er thing ;) ). Always KISS (keep it simple and stupid).

The other thing being, even assuming a water tight application has been
built, when 'loaded'/'scaled' does not automatically guarantee
performance like a well-tuned application. For e.g. you can always
optimize the 'page' weight (i.e. html size) number of html objects and
hence the number of round trips and so on.  This is altogether a
different beast.

--
Arvind