As a SP holder I want to have a choice to approve enough number of witnesses that protects security and reliability of the network so IMHO number of the voting slots needs to be at least 21. I would rather go for increasing 30 than decreasing it.
Witness counter-votes are tempting but aside of risks it introduce it also nullifies very important anti-censorship feature of the platform which lets you use your stake to produce a block. Even with very little SP you can be scheduled in block production (ever), counter votes makes that never.