The top is a wonderful thing , it takes a load of stress from the strings and turns it into work . This is what we hear. The top is not a floor so the braces are not a joist beam . When you look at the stress and the way the loads are applied , we have a series of forces.
There is a coupled force on the bridge , the string ends are pulling up and the string load at the saddle is pushing down , this applied force will rotated the bridge. Also the neck is rotating. Then to add to that there is a compressive force between the neck block at the bridge. We often see the top sink in this area because of this. Next there is a tensional load from the bridge to the tail. Another force is a variable force from the wood shrinking and expanding across the top. The braces hold the top back from this expansion , this causes the top to rise and fall.
As John Arnold points out , Martin never went below 5/16. . You can control these forces by the shape and the HEIGHT of the brace. The higher the brace the stiffer. You can take a top from great to crap real fast. I only build over 180 so I am still learning but YES YOU CAN GO TOO THIN. The sad part is it may take a few years to show its head.
The bridge rotates , and once it gets to a certain point the top pulls off the bridge. The saddle will rotate forward and you will lose your intonation and the strings go sharp. Tite bond will allow creep plus wood can also cold creep. I have many examples of failed tops. A few years ago , there was a post not too different from this. Many convinced the builder it was ok , 2 years later he was replacing that top. I wouldn't have said to redo if I didn't think it wouldn't matter but long term this may be an issue.
Of the builders that have gone this low , how old are your guitars ? What is the intonation compared to day 1 , how much is the top raised behind the bridge ? What glue was used? How think was the top and what are the deflection numbers ? What size was the bridge plate and how thin?
When I scallop , I am careful of the shape. Martin scalloped but didn't peak until it was behind the bridge. Early in my building I did this , and in 2 years the top came off and was replaced. I am not suggesting things to make more work , but if you are putting things out there and learning how to do it , there are lines that you have to be careful not to cross.
Braces do a few jobs ,
Structure
The Transverse or truss rod brace helps to carry the load from the fretboard extension.
X braces , also are structural in that they help to control and disperse load from applied and coupled forces.
Tonal
X braces are both structural and tonal , controlling the top movement to help transform work to pressure waves in the box.
Tone bars are obviously for tone disbursement
Anti Split
While this may technically be a structure issue , these braces are more to support the top from splitting
Popsicle Brace
This is supposed to help to control the crack at the extension. I agree with John Arnold in that this area would be better served with a more Tapazoidal shape than the cross grain brace we have seen and is still used. Most of these cracks from from a mismatch of expansion of the top and fretboard.
Finger braces
they are more to control the cracks in the top. They do add some mass and would be a slight tonal issue and help with some rotation but very slightly. Of the few guitars I have seen without them the inevitable pick guitar cracks were evident on both sides.
Sound hole supports
These just help the top under the rosette and rosette also helps to keep a crack from forming as the hole has exposed end grain . Rosettes , while decorative are also helping the top to keep this area from cracking.
this is just the top , and then to add to this , we can look at the top and how it is shaped. Some people dome the entire guitar , I use a different approach , and in the shape of my sides and top I create a 3 facet top , angle from the neck block to the top of the sound hole , flat from the top of the sound hole to behind the bridge and domed behind the bridge.
Adding a shape can influence the top and how the load is shared. So as you can see , there is more to a top than what the scallop is. Also there are some very good sounding guitars that have no scalloping. You don't have to scallop to make a good guitar.
The thing we have to think about , is that building a guitar is like a recipe , we all have an end result we are looking for. This is what makes our guitars different. In the end the structure of the guitar will determine the strength , how you weaken this will determine the tonal balance. I like my guitars to be balanced in the tonal regions . Not treble or bass heavy.
Another point is the creep of the glue and wood , as they will move over time . This is why I use fish glue and hot hide glue on the tonal areas. Tite bond is a fine glue but I do prefer the protein glues.
hope this helps and I am sure Mr Arnold may chime in to add to this discussion.
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That's the kind of information that makes a class really valuable, because it can't be learned any way but by direct experience, and is great if you have one like that to feel where is "almost too far". And then study the deflection under string tension when it's done so everyone can correlate that with the feel of the free plate.
The top is a fairly stiff piece of Lutz that finished out at .104 - .107. If I get a chance today I'll check the intonation. I guess that would be the be the best indicator?