It seems the problem was that quote was slow in Nu. I've fixed that, so here's the new times:
(timeit (let a '(1 2) (car a)))
ar time: 8.613 gc: 0.308 mem: 460.696
Nu time: 7.671 gc: 0.0 mem: 88.976
Arc 3.1 time: 5.33 gc: 0.35 mem: 5050.25
(timeit (let (a b) '(1 2) a))
ar time: 12.111 gc: 0.436 mem: -19278.128
Nu time: 11.438 gc: 0.324 mem: 1435.016 (apply fn)
Nu time: 8.96 gc: 0.0 mem: 125.352 (Racket let*)
Arc 3.1 time: 7.0 gc: 0.35 mem: -2124.82
Overhead
ar - 3.498
Arc 3.1 - 1.67
Nu - 1.289
Nu now has the lowest overhead out of the three...! Also note that Nu does not spend any time in garbage collection, unlike ar and Arc 3.1.
This seems to be a common trend: Nu either spending no time in garbage collection, or less time than ar and Arc 3.1. Not sure how important that is compared to raw speed, but it's nice.
Unfortunately, this also demonstrates that applying nested functions is slower than using a Racket let*. So the reason Nu won the speed contest earlier wasn't because of my destructuring idea: Nu was just plain faster than ar in general.
