The armboard on both the 20-12 and 30-12 is designed so that it can be reversed to use a 9inch arm, using the same cut-out. It's easy. From the start they assumed that some people would use the long chassis version with shorter arms. Their own advertising states that they regard the longer arms as having lower distortion.
As for other changes; well, SME are remarkably slow to boast. I didn't know until recently that, for instance, the armboards on the model 30s are made of coated brass, on the 20 they use aluminium. Even the little suspension rings are made of different material.
The 30-12 is made of thicker plates and has a number of tweaks to the suspension, including a different viscosity of damping fluid. The greater mass and wider stance makes it still more resistant to vibration. Ken Kessler's original review tried both 9inch and 12inch arms on the 30-12, and he said that it was superior to the standard 30 using either. I know he's a bit...well..but everyone I know who has tried this experiment has come to a similar conclusion. As 'Merlin' said, it's easy to demonstrate, and quite striking.
I had planned to try the Dynavectr 507 on the SME, but the company regard that as a special-order for the armboard and want about £1000. So I'l not be able to try that.