RE: XPS L521X SSD Upgrade
Dell has tested the system with maximum 128 GB SSD drive; however 256 GB SSD has not been tested by Dell.
For the BIOS settings, if you are using a mechanical drive you may use Intel Smart Response Technology to cache the most frequently accessed data to SSD for faster access.
However, as you are using a SSD drive to install the operating system, there is no need for the ISRT to be enabled.
You may install the operating system in the AHCI mode, as operating system being installed on the SSD would automatically provide you faster access.
For any further assistance with operating system installation, please share the operating system you want to install.
Keep me posted with the information. I will be glad to assist you further.