Electronic & Hydraulic Shifting (Di2, Ui2, customisations)

The differences are tiny

In Watts on a dynamometer, yes. IIRC, the difference between 48/12 and 64/16 on a track bike (straight line, bushed chain, clean and oiled) is between 1W and 2W. The feel though; all that horrible vibration gone.

As far as chordal action goes,

1. It's the principle of the thing. It can be specced (not even redesigned, really) to be more efficient, so why not?

2. Like tester says, yuck. I want my cogs to approximate circles, not polygons.

3. A bigger reason to put more metal in your drivetrain is to reduce wear. It's certainly the main reason I hacked OSPWs into my RD-9070 cage; holds like 3% more chain. Bike weighs 6.14kg btw, so not gonna break the weight bank...

4. But the main reason small cogs blow chunks is the huge gaps between them, which get comically large below 12t in relation to what's actually desirable at high speed.

If you want to maximise your speed for a given effort, you need close ratios. Otherwise you're either going a bit slower than you could, or trying harder by spinning or stomping.