The difference between what the car maker advertises and the SCPU is that you actually can reach the maximum performance in a lot of real life situations with the SCPU. Yes, there are limitations, and unofficial opcodes can never work. Still, being able to enable the "turbo" can be quite handy at times. The SCPU can also be switchen between 1MHz and 20MHz while running, in case a timing sensitive loader or similar doesn't like the faster CPU. After loading you set the SCPU to 20MHz again. There's a switch on the SCPU for that purpose.
And, as stated, there is even a mode called GEOS optimization in order to get the most out of that software (which I used to write reports for school back in the days
The 16-bit mode of the 65816 and the 16MB of RAM may also come in handy in case you feel like producing some software on your own. Heck, I even made DOOM
run on the SCPU, just because I could.