OK, I got it working.
I partitioned the disk completely from terminal.
disk0 is my internal drive, this is the one going be partitioned.
3 partitions specified, one for OS X, one for Recovery HD, and one with the remaining space for unallocated space/free space/over provisioning.
The size parameter (20G) for the last partition isn't really used because diskutil does assign to it the remaining space left unused by the previous two partitions, ie. 256G-200.1G-650M=free space.
Here is how I did it:
macbook-pro:~ $ sudo diskutil partitionDisk disk0 3 GPTFormat jhfs+ MacOS 200.1G jhfs+ Recovery 650M "Free Space" free 20G
Started partitioning on disk0
Unmounting disk
Creating the partition map
Waiting for the disks to reappear
Formatting disk0s2 as Mac OS Extended (Journaled) with name MacOS
Initialized /dev/rdisk0s2 as a 186 GB HFS Plus volume with a 16384k journal
Mounting disk
Formatting disk0s3 as Mac OS Extended (Journaled) with name Recovery
Initialized /dev/rdisk0s3 as a 620 MB HFS Plus volume with a 8192k journal
Mounting disk
Finished partitioning on disk0
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *256.1 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS MacOS 200.0 GB disk0s2
3: Apple_HFS Recovery 650.0 MB disk0s3
...still 2 free space sections ->
macbook-pro:~ $ sudo gpt -r show -l disk0
start size index contents
0 1 PMBR
1 1 Pri GPT header
2 32 Pri GPT table
34 6
40 409600 1 GPT part - "EFI System Partition"
409640 390558168 2 GPT part - "MacOS"
390967808 262144
391229952 1269536 3 GPT part - "Recovery"
392499488 107618671
500118159 32 Sec GPT table
500118191 1 Sec GPT header
... cloned the original recovery partition to the partition for that purpose on the new disk ->
macbook-pro:~ $ sudo asr -source /dev/disk1s3 -target /dev/disk0s3 --erase
Validating target...done
Validating source...done
Erase contents of /dev/disk0s3 (/Volumes/Recovery)? [ny]: y
Validating sizes...done
Restoring ....10....20....30....40....50....60....70....80....90....100
Verifying ....10....20....30....40....50....60....70....80....90....100
Remounting target volume...done
...change the partition type ->
macbook-pro:~ $ diskutil unmount disk0s3
macbook-pro:~ $ sudo asr adjust --target /dev/disk0s3 --settype "Apple_Boot"
Fsck /dev/disk0s3 ....10....20....30....40....50....60....70....80....90....100
Adjust completed successfully
...after that I cloned the booteable Macintosh HD partition to a new one on the new disk and then rebooted, went to System Preferences to enable FileVault. It worked! The disk layout after rebooting looks like this ->
macbook-pro:~ $ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *256.1 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_CoreStorage 200.0 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS MacOS *199.6 GB disk1
/dev/disk2
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *750.2 GB disk2
1: EFI 209.7 MB disk2s1
2: Apple_HFS Macintosh HD 749.3 GB disk2s2
3: Apple_Boot Recovery HD 650.0 MB disk2s3
Note: disk2 is an external USB 3.0 drive.
Best Answer
Encryption happens on-the-fly. If the data was written to the hard drive, then passed through an encryption algorithm that then deleted the written data and re-wrote it, encrypted, that would be hella inefficient, and would largely defeat the purpose. I think the answer to your question is no, but of course remember that if you're going from a data set that's not yet encrypted, then all of that data needs to be processed, so that initial process of reading, encrypting and re-writing data will have an effect.