based on your own personal experience, that tilt is of no practical value for macro photography.
Never went that far.
Just as example: You have a solid black marble of 12mm diameter full frame at f2.8 (2:1). Non tilted, its outline is sharp. As soon as you tilt, you can stop down as much as you want, you will not get the whole outline/conture into focus any longer. In addition, as was mentioned before by others, at a close range the angle of the focal plain does not change as much when tilting. So even if you cut the marble in half, you (the sensor) would still need to nearly face the surface of the cutting plain at maybe 30 degrees when tilting 20 degrees in order to get the plain into focus throughout ( I do not have any charts for that and may be off).
I just remembered that most of my examples have crashed and disappeared. Otherwise I could have sent you at least pictures of a key at appr. 1:1 and 2:1 that show how little change of perspective 12 degrees of tilt allow for when the task is to keep the entire surface of the key in focus.
I owe you examples now somehow, is can see that. But please understand that I will only post something once I find a subject worthwhile to photograph. I simply have spend too much time testing, buying and reselling lenses. I do not like it anymore.
Please do not invest too much money and be disappointed like I was. You might want to try the 8degrees that an EF to EF-Adapter allows for (ebay). Mine didn't last long and had a very stiff lens-mount. But you can check for the effect that way without investing to much.
What I came to use most beside the Pentax 67 100 Macro and a Tilt-adapter from Zörk is the Pentax 645 75mm with the life-size-converter that belongs to the 67-Macro. That way I can use the Mirex-Adapter Pentax 645 to EF with tripod food. It is more precise and sturdy than the Zörk- Adapter and I can change 645-lenses without taking the foot from the tripod or having to rearrange tilt-angle. And the 75mm is really light-weight and sharp.
But I do not quite share the enthusiasm. Diffraction is high at f22. And there are all those close-range limitations mentioned before. And mostly, you will have vignetting when attempting maximum tilt on FF.