I agree with those who suggest that you simply get started. However, to answer one of your questions: there is quite a difference between the 9" and 12" SME arms. I spent two weeks directly comparing the SME V to the SME V-12 on an SME 30/12 table using the same cartridge and cable.
Moving up to the 12" arm was like going from a highly resolving two-way compact speaker to a three-driver, three-way floor standing speaker. Extension and weight improved as expected. But unlike with some such speaker changes, there was absolutely no less coherence or sense of balance. There was just more information presented in a cleaner and clearer way and there seemed to be less distortion. IMO, well worth the cost to upgrade.
But changing the plinth and selling your unused SME V just to accommodate a new 12" arm may be more hassle and expense than it's worth right now. Check your priorities, get started, and join the fun.