I think you're underrating the difficulty of this kind of improvement. Rockets already have to be extremely well-optimized to reach orbit, so there wouldn't be

I think you're underrating the difficulty of this kind of improvement. Rockets already have to be extremely well-optimized to reach orbit, so there wouldn't be the kind of low-hanging fruit like removing 300 pounds of insulation without side effects. Also, if you want weight reductions to give payload increases in a 1:1 ratio, the weight has to be taken off of the final stage (in F9's case, the second stage); removing it from earlier stages gives you much less benefit.