First, how a yanagi is designed to cut, the spine thickness means a lot less than on a Gyuto. This is true to a similar degree for all single bevel knives. Comparing them to a knife like a Gyuto, sujihiki or Petty is not, we'll, it's not really possible.
You can make paper thin slices of a potato with a yanagi, but the cutting technique would be different.
Gyuto to gyuto, spine thickness does matter, but not nearly as much as a great grind. I can make a knife with a 3.5mm spine still cut like a laser for the most part. Where you'd run into trouble is if that thick spine had to go through something.... like a pumpkin or something.
Small screen on my phone so I can't even properly review what all I've said, hope it all makes sense.