I've got no experience with sandbox, primarily because I do everything through slow growth. I'm concerned that this may change. I've got a site that is older, ranks reasonably well, and has good/great backlinks. But it only has about a dozen pages (that's all that's been needed to make the service available). Now however I'm about to launch a project on the site that will involve about 10K original content pages to start. Then I'll sporadically be adding a few hundred or thousand new pages every month or so. The intent behind the project is to develop links - people will link to the content. I expect to get reasonable number of high quality links (authorities and edu's in my niche) as a result - that might also be a shock to the site in an industry that doesn't see many .edu's links. Matt Cutts has stated that adding millions of pages might be a problem, but that thousands shouldn't. Do I believe him? Or are my rankings that are on the verge of going to the top as a result instead going to see my site disappear?
Just a Wanabee here but My mall site is in the Sandbox and its been some time now I wish playtime would be over ..
If it's any help, one of my sites went from xxx pages to xxxxx pages without any problems from Google. At the end of the day, it's got to be your own decision. If you're really worried, maybe you could take your site from xx to xxxxx pages in several batches, only adding another batch of pages after the previous has been indexed. Once you've safely reached xxxxx pages, you'll be able to add your few hundred each month without it representing a huge percentage increase in the number of pages.
If current site has great inlinks, and the new content is original and has value then It should not create a problem.