12 percent increased chassis stiffness and a 3 pound heavier platter would probably make the stretch 30 audibly better in the bass foundation of the music over the standard 30. I have never been impressed by the 12 inch arm concept. The 12 inch kludges designed for the original massive broadcast tables were not intended for improved tracing accuracy, per se, just convenience, cuing, and use with the old style low compliance cartridges. The improved "tracing distortion" has always been an isolated factoid taken out of context for the purpose of justifying a nostalgia craze and I seriously doubt it is audible outside of the self anointed who believe they can hear a gnat fart in a hurricane. A "27 percent decrease in tracing distortion" translates into perhaps 1.1 percent tracing distortion at the distortion apogee vs. 1.5 percent tracing distortion with a nine inch arm. It is more likely that any differences in sound between 12 inch and 9 inch arms are based on mass effect and different compliance cartridges.
I have found that the Da Vinci Grandezza arm extracts much better bass and lower midrange from the standard 30/2 than the SME IV-VI arm, so bearing dynamics of the tonearm seem to contribute as well as turntable mass effects.
Nontheless, the stretch 30 with a 12 inch Grandezza arm would have to be a killer diller combo. SME has also made the cost increase very reasonable considering the increased weight and stiffness of the turntable/platter in relation to the standard 30.