. . . pretty much none. If I'm understanding his setup correctly, the cost of overboots and related custom hardware will save a very small amount of money over some used xc classic race boots and bindings.
But his setup "works" with trail runners (or whatever non-snow footwear is being used) only by slipping them into the overboots (which are permanently attached to the skis). You could instead just slip off the trail runners and then put your feet into the xc ski boots for a far more efficient and in-control skiing experience (which I think anyone would agree adds up to more fun).
The only advantage the screwed-on overboots have is a very slight weight savings (i.e., a few ounces) relative to xc classic race boots & bindings. But the striding inefficiency and lack of skiing control will more than offset the weight savings.

I mounted the heavy-duty Voile 3-pin bindings on the Hoks and used my only 75 mm Nordic Norm boots, an early pair of Scarpa T1s (similar to recent T2s). The skis climb great and glide surprisingly well. They are very easy to turn. However, the factory binding position is quite a bit too far forward for those accustomed to skiing. I think they are meant to appeal to reforming snowshoers. I made some light-weight adapters of redwood that move the bindings back three inches, which allows comfortable linked telemark turns, or anything else you want to do with the Hoks.

T1s are overkill for these short skis, so I ski them unbuckled, which makes them very comfortable indeed. The boots have Thermofit liners, and that makes them slightly lighter. I will try to find a smaller lighter 3-pin boot for these skis. Or I may mount some adapters I made to fit my SNS bindings and use some Karhu SNS boots on hand. That would be a fairly light setup overall. But even using the T1s, the rig feels pretty light to me, accustomed as I am to a very heavy backcountry telemark setup.

It is no surprise that these skis are at their best in my "backyard", which is the foothills zone (7-8.5K') south of the San Juans and east of the South San Juans in southwest Colorado. There are lots of up and down, lots of brush and trees, and usually thin snow cover. As noted in the review, the built-in skins must be kept well-waxed and scraped from time to time, if they get iced. This will happen when skiing alternately on sunny wet snow and cold shady snow. I am seeing signs of wear on the skins. Eventually, something will need to be done, but probably not for a few years.

I have tried skiing them up high in the Wolf Creek Pass region, and that is the wrong terrain for them. They are not meant for big descents where maximal glide is best. There's no harm in trying though.