I am a big time metric person as well so I want to say this. Make sure you know what you want to measure and why.
Grams per watt is one measurement but what I don't like about it is that it doesn't include time. For instance, I can veg 4 plants for 3 months and harvest it, get 800 grams out of 600 watts and have a nice average of 1.33 grams per watt. Great right? How does that compare against a crop grown and vegged for 50 days instead of 90 and yields 600 grams. That one is only 1 gram per watt. Time has to be a factor.
The issue with measurement people is that they need to ensure they are measuring the right thing. Show me how I am measured and I will show you how I perform. If I am measuring Grams per Watt, what stops you from vegging incredibly long periods for larger plants, specifically to measure well on your metrix. The reality is that Grams per Watt doesn't paint the whole picture. I used to measure Grams per Watt, actually still do.....but I do it in conjunction with other metrix over time.
I stole Grams per day from Graytail. Gives me an indicator of the performance of the plant. My highest yielder was a Northern Lights that I vegged for 70 days and flowered for 65 and came in over 3 grams per day. Most of my other plants are closer to the 1.4-1.6 grams per day. I also like to track grams per gallon of soil as well.....kind of gives me an idea of what size pot used in conjunction with how long I let them go, gives me the best yield over that said time.
For me, efficiency is how well I use my space and time to produce the most of the best produce I can. Its not just about grams per watt. Its a combo of grams per watt and grams per day and grams per gallon. I figure over a couple years, I will know what strains grow best in what size pot, for what amount of time.....FOR MY setup. Each person's will be different based on their individual growing environments (RH, temps, lights, medium, etc). What works for me, won't work for everyone.