Yes it's(the XL) the same switch with more ram and faster cpu basically. VPC is possible between two totally different switches, but whether Cisco locks this out of the software (i.e. if the VPC code checks the model number of the switch and won't form if they differ) I can't say. Technically though, mlag/vpc is possible between any two devices. As with anything , test it first and see. If it forms up you should be good , and since they are the same hardware I see no issues.
