I just sent you an e-mail about this, but here's the general idea:

Steve recently made some changes that allow us to link to projects outside of the dfdl repository (DFDL-753). If you create a symlink in the daffodil-extra directory to another project directory elsewhere, this project appears along with the others in sbt. Steve mentions in the ticket that this could be used for the performance data that does not fit in the repo. This could be the proper long-term solution to how we tie our svn repo to the rest of the performance data. Please look into this when you have the chance and see if it's a feasible solution.