I think you're better off getting the "sandbox clock" to start ticking as soon as possible.
if you can put some content on there that is not "page coming soon" that is best. Just take 30 mins and create some content about what is going to be on the site, purpose, mission, yadda yadda. Maybe contact info, if appropriate.
Submit, get the clock ticking then develop the "real" site as soon as possible.
And it's not only a sandbox issue - Google need some time to update your backlinks, and calculate PageRank, so if you put a few inbound links to your new domain, you may already got some PR when the site's ready to be opened.