SLA values of JIRA like AssignmentTime, TimeToResolution etc are stored in customefield_$number$ fields. These fields can be searched using the jira rest command with the changefield option. Following is the example search.
In the new Add-on for JIRA 2.2.1, these values also come through when using the normal |jira jqlsearch "<your_jql_query>" command. However out of the box the output is very ugly and difficult to deal with, please see https://answers.splunk.com/answers/560281/add-on-for-jira-221-and-220-not-parsing-jira-field.html?ch... for how I modified one of the commands to get better output. In it I chose to have the SLA values come back as 4 subfields of each SLA (elapsedTime, remainingTime, goalDuration, and breached) since there are really more than one thing you would want to know about the SLA than just elapsed time (the value is in milliseconds).