Efficiency was mentioned a few times, but without any hard numbers. What was the state of the art efficiency over time from a 1940s design to today? Have improvements plateaued or is there still a lot of theoretical gains on the table?
Thermodynamic efficiency (which is what turbine blades enable) has increased from ~30 to 55% over the last 50-60 years. The book estimates that the practical limit of for thermodynamic efficiency is in the 65-70% range.
All other things equal, range varies proportionately with efficiency. Ie, if you have 500km range at 50% overall efficiency, then at 65% overall efficiency, you have 650km range.