Just my 2c here. Have not stripped any guitars (lately) to refinish in lacquer. Have not done any tests dealing with finish thickness. Just plain ol' engineering oriented moi.
Thin finish - old finishes were nitrocellulose lacquer. It shrinks. It shrinks continually over its life. Nitrocellulose - look at celluloid pickguards. They curl. They warp. Look at any on old archtops - they go to pieces. Old finish checks. Nitrocellulose based film is dust now. I've played with nitrocellulose (celluloid) film, back in the 70's. The film substrate (base) gets VERY thin, curly, wavy.. before it turns to dust. When nitrocellulose lacquer is applied to a surface, whats left over after the solvents dry, is nitrocellulose. So thats what happens to the finish. Its my belief that a "thin" finish on todays instruments is nothing more than a recreation of what time has already done to the finish on old guitars. I really dont think it effects tone all that much, if at all.
Tone - my tests, limited as they were, but none the less conclusive, having put different necks on guitar bodies with the same pickups to see what happens with tone. The bodies were light ash, hard maple, thinline mahogany with maple cap, thinline mahogany with walnut cap. At one point they all had at least one pickup in common - from the now discontinued "52 RI" pickup set by Fender. The necks were solid goncalo alves, hard maple with pau-ferro, solid hard maple, and mahogany with rosewood. The number 1 tone shaper - the neck. Using the different bodies on the same neck, gave some difference in tone, but nothing like using different necks on the same body. In my way of thinking - this stands to reason - because the most resonant part, the part that wiggles the most from the strings vibration, is not the body, but is the neck. Solid vs thinline, sort of hard to say conclusively because there were wood differences, but the thinlines were (are) not as bright. Hard maple vs ash not as much difference as you'd expect, and really ash vs the thinline mahogany bodies was more like variations on a flavor, not different flavors of tone.
This brings me to - why would a finish alter the tone? To alter the tone, the timbre... we alter the overtones, which are harmonics. Some harmonics can be suppressed (attenuated), leaving others to be more prominent. There is some evidence that a standing waves can occur for certain harmonics, giving the observed effect of more prominence... even though its more of a psycho-acoustic thing than measurable. IOW, its how our brain interprets what is happening. By changing the resonance of the various parts, we alter the overall way the various overtones are either canceled or not, or even pseudo multiplied through standing waves.
I remember doing tests - structural tests but they apply here - basically looking how surf boards are made, and how that would apply to wings, or other structural parts. When we look at a surf board, we can see some really light weight components that act together to make one really strong assembly. That is, the very light foam core, with no outer material on it, is very soft, very limber, and easily bent. But, add some glass cloth and resin, and you have got a laminate. The glass cloth - or I suppose something better than that today, like thin kevlar or carbon fiber - is bound to the core with the resin, which is also somewhat brittle, but has has high adhesion to the core. The result is the sandwich of parts is far stronger than the individual components. This is the effect of lamination - it makes things stiffer when you apply an elastic material to a non-elastic core.
Let me remind everyone that elastic in this sense is not like a rubber band. In this sense, elastic is like a golf ball, which is very elastic, versus a ball of putty, which is non-elastic. The wood is less elastic than the lacquer. The foam of the surf board is less elastic than the glass/resin coating.
You can see where I'm going with this... I hope.
There are far far to many tone shapers to accurate evaluate and predict the overall tone of a guitar - on paper - from looking at the materials and specifications. We rely on experience - the knowledge gained by what already was tried, and we experiment, constantly refining our ideal model - which may only exist in our mind, and not actually be achievable.
Does the finish make a difference. It has to. But I will say - there are far far far more important things - and besides pickups (and hands) the most important tone shaper is the neck, its material, and profile, when dealing with solid body guitars. After that - solid vs chambered or thinline is the next biggest. After that... they're all pretty far down the food chain, and certainly, after hands, pickups, neck and body... far down the list is choice of strings, and even more remotely out in the periphery of things that effect tone, is finish, how thick, what kind, and how stiff.
As a final "aside" to this discussion - back in the late 80's or early 90's Fender produced a terrible instrument in the far east. I think it was Taiwan or Korean made. It used a very light and weak wood for the body. They used the finish to produce a structurally strong instrument. I cant remember the wood, it was some far east wood, not Nato wood, but something far worse. The finish was polyester - applied very thick and cured in a way to make it rather stiff and tough. The guitar had the strength to hold the various components and not break. It sucked. But its an example of how laminates work.
Certainly more change in tone by changing string brand and type than in the finish of the guitar - and I mean no slight on Tonar's experience and expertise. I'm up for lacquer any day, but also go for the epoxy - like this next one will have.