Duncan was never more efficient than fully developed Boogie.
I'm sorry, but this is just not accurate. We don't even have to make excuses about Cousins being misused relative to Duncan, because there are a lot better numbers now that are a bit more telling than a mess of overlaid red and green on a shot chart.
Let's just look at this claim, using the numbers.
"Duncan is not more efficient than Cousins"
Duncan is more efficient in each of the four inside-the-arc distances (0-3 feet, 3-10 feet, 10-16 feet, and 16-23 feet) than Cousins. Interestingly, Duncan is a slightly better shooter from 16-23 (.405 vs .385 career), and the disparity gets somewhat worse as you move in (10-16 feet, .403 vs .343; 3-10 feet .442 vs .352; 0-3 feet .695 vs .629). From the preferred regions Duncan was clearly superior. From 0-3 feet, Cousins' best year was a .673 percentage, but Duncan was .695 for his career and had only two of his sixteen seasons less efficient than Cousins' very best year under the rim. It's just as bad from 3-10 feet, where Cousins' best year was .402, and Duncan (.442 career) again beat that 14 of 16 times. In fact, the only position on the court where Cousins can be said to be more efficient than Duncan is outside the three-point line.
"But Duncan wouldn't be more efficient than Cousins if Cousins shot more from the inside, like Duncan"
Actually, Cousins shoots more from the inside than Duncan ever did. Cousins averages 37.5% of his FGAs under the rim. Duncan only beat that number once in 16 years and his career average is 30.9% of shots under the rim. Duncan does shoot a bit more from 3-10 feet than Cousins (33.4% vs 29.6%) but Cousins' efficiency in that range is so bad (.352 FG%) it's hard to argue he should shoot more from there. As for 10+ foot jumpers, Duncan has taken 35.8% of his career shots from that range, while Cousins has only taken 32.9% of his shots there.
"But Cousins was more efficient under Malone because he took more shots on the inside"
Cousins is most efficient directly under the rim. But under Malone (2013-2014 season), he only took 35.9% of his shots there, below his 37.5% career average and only barely above this season's 35.2% of his shots directly under the rim under Karl. In fact, it appears that the threes that Cousins is taking this year are coming almost solely at the expense of his shots between 3 and 23 feet.
"But Cousins shouldn't be taking so many threes, it's a terrible shot for him compared to a closer shot"
Cousins has developed his three pointer into a fairly efficient shot this year. He's getting 0.924 points per shot from three, which is better than he has ever done in any season at any distance 3 feet from the basket or more, and it's not particularly close. Excluding threes, Cousins' efficiency over his career is 0.726 points per shot from three feet and further.
"But Cousins being out on the perimeter more is hurting his ability to get to the line"
Cousins took 13.2 FTs per 100 possessions under Malone in 2013-2014, and he's taking 13.7 FTs per 100 possessions, the best rate of his career, so far this year under Karl. (By the way, Duncan's career mark was 9.6 FT attempts per 100 possessions, and his career best was 12.5, so Cousins is way better at getting to the line than Duncan ever was, no matter whether he's playing more on the perimeter or not.)
Bottom line, Duncan was way more efficient than Cousins at anything that wasn't a three - this is despite not getting to the line as much. Cousins' lower efficiency this year is actually due to poor conversion under the rim (.586 FG%, well below his .629% career average) despite a very similar number of attempts there, and not due to his three point shot. Why has he lost efficiency under the rim? I'd guess it's a lack of lift resulting from aftereffects of last year's viral meningitis and the various injuries he's dealing with this year. Even then, at his best he has been able to be more efficient than only the very worst seasons of Duncan's career. Cousins is a great player - he's an incredible rebounder, an underrated defender (and the best charge-drawer in the league) and he's very versatile, having quickness, ball-handling ability, and jumpshooting rarely seen in a player his size. But he's not remotely an elite-efficiency offensive player, and he never has been.