I finally did the comparison between an SC farm and a multifarm.
In this corner, measuring 17 x 17, is the extra large multifarm:
And in this corner, measuring 16 x 16 is the Steve's Carts tree farm:
The methodology was simple. I started the timer when the cart unloaded its previous cargo. 35 minutes later, after the SC finished unloading it's second lap, I tallied the totals. The results:
The multifarm had a pretty big advantage in apples. This is because, when you plant regular oak trees in a multifarm, they drop "apple oak" saplings, which have a much higher apple production rate. You can't even plant apple oak saplings in the SC farm. But obviously, the SC farm had a sizable advantage in the more important categories of saplings and trees.
For the record, a full-sized HP boiler requires 3802 planks per hour to maintain 1000 degrees. In just 35 minutes, the SC farm made the equivalent of 4608 planks. The multifarm made the equivalent of 3468 planks.
EDIT: I should also point out that the multifarm made 97 blocks of sand and presumably used 97 blocks of dirt in the process.