To understand how to use a wind shirt properly you either need a lot of experience or an understanding of the related physiology and physics concepts. I can't help with the experience option but, I can with the other option.
"We are talking about a very thin wind jacket"
-Thinness is related to weight but, it has NO correlation with the primary functions of a windshirt. The primary function is to maintain an optimal micro-climate for a broad range of conditions.
"I only put mine on because I want to be warmer"
-In the typical UL backpacking environments the optimal micro-climate for comfort can change quickly and frequently. For example: in the shade; out of the shade; sheltered from wind; not sheltered from wind; walking on flat ground at 3 MET; going up a hill at 7 MET; sun behind clouds; sun unobstructed; ground hard; ground sandy; ground covered in sun cupped snow; radiation from a rock out cropping, no radiation from rocks, etc..; and etc.
"If I then get too hot, I either unzip it or push the sleeves up. Or, take it off."
-You have defined your way, the most inefficient, to deal with the micro-climate variability. The problems with that approach are twofold. First, taking a jacket on and off to regulate temperature, while carrying a backpack, is the most time and energy inefficient scenario possible. Second, the scholarly research has shown aperture ventilation (unzip, zip, push up sleeves, open pit zips, etc.) achieves less reduction in moisture transport resistance than an appropriately air permeable fabric.
"I have other shirts that are nearly 100% breathable. For example, virtually any long sleeve tech shirt will provide a little additional warmth with virtually no wind protection. Many of my long sleeve shirts have large mesh panels with terrific breathability."
-100% breathability is also known as the garment air permeability required to never become an impediment to sustained physical activity. That value is 400 CFM; it is typically only found in a loose weave T-shirt. Most conventionally woven technical shirts have an air permeability of approximately 100 - 150 CFM.
The scholarly research shows that for a base layer plus windshirt ensemble, as the air permeability goes up, the water vapor moisture transport also goes up approximately linearly to about 35 CFM. After that, the ensemble's moisture resistance elements found in the base layer, the base/windshirt air gap, and the windshirts’ boundary layer predominate. Further increases in the windshirt's fabric air permeability have a negligible benefit in the ensemble's system performance. Since a more breathable windshirt allows more forced convection (wind) in, most people intuitively believe they are improving their moisture expulsion rate; they are not.
"... there's any easy solution. Just run any of these jackets through the wash a couple dozen times to get rid of the DWR finish. I suspect that will make them quite porous"
-Any commercial DWR finish, applied according to the manufactures directions, has zero effect on the breathability of a garment. Washing doesn't change the weave and so the air permeability stays the same.
-UL backpacking is the only common sport that produces 7 MET activity and its attendant heat on a sustainable basis. It is the case in which most windshirt manufactures don't specifically target. Those that do, size the windshirt to create an optimal gap between it and the base layer of 3/8" - 1/2" adds .6 clo of warmth for free. They provide a weave that passes approximately 35 CFM because this the level that provides the most exports of body moisture with the least vulnerability to forced convection heat loss. Those windshirts optimize for UL backpacking also have either an EPIC thread coating for garment life time DWR or a fluorocarbon coating for moderate life DWR.
A windshirt designed for UL backpacking will best maintain a thermal neutral body temperature without requiring the wearer to constantly diddle with it. It will also yield a HH value in the range of .43 – .75 PSI (300 - 527 mm H2O HH).
In summary, a properly designed windshirt for UL backpacking adds warmth by the addition of .6 clo in the base-layer-to-windshirt air gap plus another .6 clo from the windshirt's boundary layer (this will vary with wind speed). When your body goes above its thermo-neutral point, you start producing sensible perspiration. If the air permeability allows the moisture to evaporate and pass through to the outside environment, the latent heat of evaporation cools your body back to the thermo-neutral point. If the air permeability is not sufficient to pass the amount of moisture being produced to the outside environment, the water stays on your body. Once approximately 20% of your body is wet, you have a feeling of discomfort. This is primarily sensed by the increased clothing friction. To put the difference in ability to pass moisture out in perspective, a Frog Togg or equivalent garment will pass .17 CFM and a pre-2013 Houdini will pass 35 CFM. Hence, a dramatic difference in the ability to automatically cool your body as required by different brief micro climates without any diddling.