Where have you been? I posted performance benchmarks based on that a week or two ago. See all my updates to LOG4J2-1359.
I have already implemented StackWalker support on branch java9NoMultiRelease. I started on a branch for LOG4J2-1359 but trouble with the Multi-Release jar made me rethink it so I reimplemented it on the new branch. Once I have support for modules I will be merging it back into the LOG4J2-1359 branch and looking for a code review. In the meantime you are free to look at the java9NoMultiRelease branch.