Does anyone know why and if there is a performance difference between encrypting the drive on a clean install versus after things have been been written on? Curious why there's a difference, if at all...
I don't know if there is a performance difference. The issue is that it might be easier for someone to dig up data from the SSD.
Also I've read that there's a performance drop with using File Vault2 with the SSD, which is why I've opted out of encrypting for now.
If there is a performance difference, I hardly noticed it after transitioning from a 2010 MacBook Air (Toshiba SSD) running Snow Leopard to a 2011 MacBook Air (Samsung SSD) running Lion and FileVault2. I think AnandTech ran some specifications and estimated a 15-20% difference on some extended read/write operations, which likely isn't noticeable, particularly for someone coming over from a hard drive. The difference might be more pronounced on an older MacBook Air, since the Sandy Bridge chips have special CPU instructions to speed up encryption that the Core 2 Duos lack. If you are running a 2011, I'd just go for it now.
If/When I choose to sell my MBA, I was wondering if I could do the following and get the same results as if I had kept my drive encrypted all along:
1) Wipe SSD
2) Reinstall Lion
3) File Vault2
4) Wipe SSD
5) Reinstall Lion
6) Ready to ship
Would this work?
I think that would help, but it wouldn't be as secure as having encrypted data from the start. I think the issue is that it is possible that the wipe process won't really wipe the SSD. In other words, the SSD's controller will just report back to the OS that the particular SSD block has been overwritten, when in reality it hasn't been, leaving unencrypted data "underneath" that can be picked up later.
Stated otherwise, that would likely stop a casual "hacker" from recovering information using an off-the-shelf utility, but it likely wouldn't be enough to satisfy enterprise/governmental requirements.