From the manual…
Note that very high resonance values might cause the filter to become unstable and produce unexpected sounds, especially in combination with the overdrive parameter.
It does indeed become unstable and produce very unpredictable sounds. Even more so when poly and unison enter the picture. Extremely satisfying, sounds like a whole new machine 
Really excellent upgrade. I still wish the note slide behavior could be adjusted and a few other tiny user interface things (eg LFO destination still easy to change by accident) but these are very minor problems compared to better patch/kit management, better filtering, better voice allocation, better storage…my A4 feels like it just doubled in power.