Different strategies:
"Instant" generation of metadata then
A) before updating production block copy it away to new location
B) put new production block in new location
(maybe assign an empty file and gradually fill it).
This "copy" could be something like SRDF to a remove device, with a "split mirror" operation at the remote.
I used an ISO to install Delphix so I selected Sun Solaris 10 from the VMWare list of O/S options.
At the time of speaking (Sept 2014) the latest version of Delphix is 4.2
It may be possible to store completely empty Oracle pages in the metadata entry of the block.
The Delphix-driven rman backups are "from SCN" - Delphix keeps track of the SCN reached at its previous rman call. The code takes steps to ensure that the Delphix backups don't cause confusion in the rman catalogue if you are also using rman as your primary backup mechanism.
Snapsync - for incrementals, you could do two one after the other if the typical incremental is slow: the second incremental will be small, applied quickly, and allow for faster provisioning.
Pre-Provisioning: pre-provisioning applies redo necessary to make a snapsync immediately provisionable, ahead of time. Allows for constant time provisioning in a few minutes, regardless of database size or change rate.