How to get my lost Xperia SL back even after factory reset?
As soon as the device gets factory-reset, you can forget about tracking it with Google's Device Manager: As your Google Account on the device is gone then, there's no way for Google's DM to communicate with it.

Unless you've installed some anti-theft solution which integrates with the ROM (and thus survices a factory-reset – a feature offered e.g. by Cerberus, if your device is rooted), you will have lost all control then. But even a ROM-integrated solution would be gone as soon as a new ROM is flashed.

As long as it's still working, you could take a second device and follow the lost one. Once in range that you should be able to see it, you could trigger the 5-minutes-permanent-ringer. In the hope for an "honest finder", who should take this as a proof and hand your device back to you.

Don't wait too long, or the battery might be dead!