We have done some tests, but we haven't created a report for this yet. This is something we will do in the near future. From our tests we found that eZ publish scales well if you do not put very many objects in the same "category". E.g. a site with one category and 50000 objects in that category is much slower than a site with 100 categories and 500 objects in each category.
We will publish some numbers as soon as we have a report ready.
