Some initial feedback on this topic.
This issue was highlighted to the XTU development team on May 7, 2015. Our report was based on findings of user @henkenator68NL who provided us with the test data via @HiVizMan. Based on the findings, the XTU development team added a fix in v6.0.2.8 which prevents a user from changing the priority of the benchmark process or any other XTU related processes. This version was released in October 2015.
Based on this thread we'll check if this issue is still present on the latest XTU release and if so, check with the XTU development team for more information.