Currently there is no means to promote a subsite to a site collection and choose the following options:

1. Number of versions to migrate - it takes everything!!!
2. Which libraries to migrate - it takes all of them!!! Including the preservation hold library.
3. Replacement for orphaned users, such as 'system account'

Not having these features in place has resulted in a 300Gb site with hundreds of versions above the configured limit due to retention policies being copied over - it was only attempted once and abandoned via this method because of these limitations and more importantly because it appeared to fail for no discernible reason after several hours - the mouse spinning around the wheel with no impact to the destination site - the task was aborted without knowing why it stopped.

Thanks,
Jason.