Looks like you already pulled the trigger but I'll weigh in...
I think the #10 is closest to the 110 in overall size, including the handle.
I think the #9 is closer to a large sodbuster in overall size.
I think the #8 is closer to a 112 in overall size.
Two important notes.... As with axe handles, the Opinel joint/handles get stronger the thicker they are and the better aligned the grain is. As with an axe/hatchet, if the grain is horizontal it's possible to simply break the handle of an 8 under hard cutting. I don't have the hand/arm strength to break a 9 or 10.
Second and more importantly, don't test the lock strength of the Opinel as it's a very good way to amputate your fingers. The "lock" on an Opinel is more like a stiff slip joint. It prevents the blade from moving from a casual bump but should never be relied upon under hard closing forces. It will simply pop off and the knife will close. I pry with my knives but never in the closed direction, so I don't consider lock strength a part of "hard use", which is why I asked how people define "hard use".
Oh.. third comment... any knife can be broken including the Opinel. If you want to break an Opinel quickly, pry with it repeatedly. Eventually you'll work harden the blade and will break the blade. This will happen long before the knife develops any blade play in the joint, at least in my experience.
Keep us posted. Very interested in what your experiences are.
You forgot another very important point;
The whole structure of an Opinel is dependent on a pin through two thin 'fingers' of wood that support the blade, bolster, and locking collar. Everything is anchored right there on that single pin. If you by accident or design apply any twisting or sideways torque on the pivot area, one of the supporting wood fingers can crack, rendering the whole knife a floppy candidate for the trash can. I've had this happen to me on a number 8 Opinel. I have witnessed a number 7 I think it was, many years ago break in such a way the whole metal structure sheered off under some heavy cutting. Granted the cutter was a 6' 4" inch 240 pound strong guy who was a bit stressed that his wife had a broken leg, so YMMV. But Danny was forever after, never without his SAK that had a saw blade on it.
I had about a thirty year love/hate relationship with Opinels, modding them, sanding and staining the handles, soaking is linseed oil or melting wax into the joints to try to water proof them. Hammering the pivot pins to tighten them to use as a friction folder, filing the locking ring to make them more secure in the locked mode. But I finally gave up on them and now I still own one, but it's in the kitchen drawer and used for very fine work in the kitchen. They do slice extremely well, but for a knife that I can drop in a pocket and go on with my life without extensive tinkering with, I just go with something else. A alox SAK, a Sodbuster style knife with real steel liners and handle scales of a durable synthetic. If I need "hard use" I will use a fixed blade. The original hard use knife.
Alos, keep in mind on that video, the Irish guy is cutting willows. This wood grows in wet ground and is soft and very easy to cut. What he is doing is bending the branch down to such a degree that even a slight cut into the grain of the wood at a 45 degree angle will cause a stress fracture of the branch. Take careful note on the video how the branch is many times actually breaking off with a long trailer that he twists off. You can do that on a lot of willows with a little Victorinox recruit. Same for notching around the base and making a V grove and causing a stress line to break off. His other video similar to this he uses a little two blade pen knife to demonstrate this. It's a little deceiving and slight of hand if you don't know what he's doing. He's a really big guy and he's mostly breaking the branches off after creating a stress fracture in the wood with he knife blade.
And like Pinnah said, don't ever trust the lock on an Opinel. It's not really a lock like we think. It's just a slight added feature to what is essentially a friction folder. For the first 65 years of their existence, from 1890 to 1955, there was no lock at all on any Opinel. They were strait up friction folders. A lawyer for the Opinel family created the 'Viroblock' for so called safety, actually making them more dangerous because some people will actually think it's a real lock. Always use an Opinel like a slip joint. I got to where I'd actually just take off the locking ring and peen the pivot for a snug fit.
