You can waste time tweaking CPU lightmass to get slightly better results with significantly longer rendertime, but still quality which is quite inferior to GPU lightmass. CPU lightmass uses irradiance caching and photon mapping while GPU lightmass uses path tracing. There is a reason no commercial renderers these days use ancient techniques like IC and Photon Mapping, at least not by default. It produces inferior results and takes longer. Unfortunately, CPU lightmass is now literally almost decade behind technology level adequate to today.