Its going to be very difficult to optimize MIRA further.

On taking the reward share from the witnesses, I think its very difficult to gain traction. Steem as a platform is becoming home for many dApps with SMTs I feel the content generation will greatly give way to applications like drugwars, steemhunt etc. ie the rewards are not going to be only for creating unique content and also for "interacting and engagement". So my take is the direction of the platform is changing a bit and the witness servers will see further traffic from the increased operations. I feel this is the ultimate goal behind the change in reward behavior and reason for keeping the pool for witnesses untouched.