To answer the OP's question, XL edition will only contain everything in the first 2 DLCs as noted in the Official US website.
http://us.gran-turismo.com/us/news/d6186.html
The weird thing is...I've looked up on the Spec II disc released in Japan, and it only mentions the codes for Complete Racing Pack, which excludes Car Pack DLC 2. Is this the first time that JPN buyers gets the shaft and the US edition getting extra freebies?
http://www.gran-turismo.com/jp/news/d15461.html
So no...for those who are hoping for DLC 3 with the Aventador, sorry to burst your bubble, but it ain't free with the XL or the Spec II edition. And to the above poster, please read the "Update" in the GTPlanet News link.