Science might argue that gross efficiency can only be measured using gas exchange analysis, but I'd trust human feeling to easily cross ref perceived effort with swim velocity to self assess efficiency. Nice path you're taking.
It would obviously be interesting and fairly easy to study on that. All you need is a flume and gas exchange machine, swimmers. You ask them to self assess according to a grid of levels, and then compare with the actual efficiency.
Something which is probably even easier to self assess might be propelling efficiency. Not sure if you came across it but I put together a little google doc to this effect
It calculates swim pace based on wingspan (or standing height) stroke rate and stroke efficiency level. By slightly tweaking rate and efficiency %, you can estimate yours out of say a 1500m performance, or like you point out (and I so much agree), any swim weather it's aerobic fruity pace, higher end competitive pace, etc. Not everyone swim with the intent of making every rep as fast as it possibly can be.
I'm amazed to see that in spite of being totally open for edit, publicly, the google doc remains top shape. People are respectful.