Surely, there is no such thing as the efficiency of outdoor versus indoor riding. It all depends on circumstances – frequency of stop signs / intersections; extent of downhills; wind direction and strength; and the like.
But it is quite easy to examine particular rides and to compare their efficiency with a given TR workout. For example, yesterday, I was scheduled to do Red Hill + 8: 1:30 hours, 111 TSS, comprising 5 * 6-minute intervals at 115% of FTP, plus warm up, rests and cool down. I tried to replicate it outside [wahoo, so following learned instructions rather than being pushed onto garmin]. This was a rural route, not too many stop signs / intersections. with slight elevation changes, but a wind. Result?
[A] It took 2:00:59 hrs to get 113 TSS. Efficiency outside ~ 75 per cent.
[B} Rest intervals were generally longer than prescribed [waiting for a rise / flat section / turn into the wind], so heart rate in zones 4 or 5 [> 85 per cent of HR max] only 00:25:25 hours, compared to ~ 00:30:00 hours indoors. Efficiency outside ~ 85 per cent.
[C] On one interval, I mis-remembered the road, and there was a stop sign in the middle of the interval. On another, I mis-remembered again, and there was a downhill / with the wind section where it was hard to maintain the power target. Two intervals messed up, out of five. Efficiency of interval completion outside ~ 60 per cent.
And so on. All you have to do really is to examine a couple of your rides in this manner to see what is really going on here – loss of efficiency, depending on your measure. Based on experiences like this, I usually reckon that an outside ride will take 30 - 50 per cent longer than an equivalent inside ride. But on some rides, with lots of stop lights, it could be far greater.
The point is that it is easy to actually identify efficiency losses outside – though the riding might be nicer [which become more and more important, the longer and longer the workout is]. You don’t need a study of 1000 riders to identify this with data: just look at some examples. And remember that each TR workout: outside ride comparison is unique.