The big issue with the RadeonSI 2D performance for any HD 7000 series GPU or newer is that it utilizes the GLAMOR library for doing 2D hardware acceleration via OpenGL. GLAMOR still has to be tuned as does the RadeonSI Gallium3D driver itself. Phoronix reader Niccolò Belli pointed out some RadeonSI performance improvements that are in development and testing.
One patch he tested and pointed out in an email to me is for speeding up glamor_copy_area for certain use cases. Another is for handling diagonal lines. The first patch fixes a long-standing bug with the KDE raster back-end performance on RadeonSI.
Belli (a.k.a. "darkbasic") additionally wrote, "The second one mitigates the GTK primitives drawing issue and completely fixes the Calc scrolling issue (previously opening some sheets in Calc took 30 seconds and scrolling was almost impossible). Unfortunately the second fix is not going to be merged becuase it isn't the proper way to do it. I don't know about the first one. Anyway we finally have 'decent' 2D acceleration on radenosi."
Additional RadeonSI 2D performance issue details can be found via this FreeDesktop.org bug report.