Just wanted to follow up on my own question for progeny.
After some more measuring, I went ahead and purchased the 4.5mm mat for my Dual 1229 and use it without the original rubber mat. It runs a AT15SS cartridge, with the original beryllium cantilever super shibata stylus assembly, into a McIntosh MA6200. I am a mechanical engineer, and my equipment was restored by me, well set up and with all functions performing to the original specification. After using the mat exclusively for about a month, I went back and A/B tested with the original rubber mat, with Bobby Hutcherson's 1971 album "Head On".
The difference was immediate. Herbie's mat does a great job dampening the resonance of the platter, resulting in a much cleaner presentation of the vibraphone. With the stock mat, some of the notes would bloom. Herbie's mat eliminated this distortion. The well-recorded double bass also benefited from this effect.
Noise floor is lowered as well, and the well-recorded percussion is the greatest benefactor. Recorded ambience is more noticeable. Each note gained tactility and the soundstage is more focused and wider. Rumble is now well below the baked-in rumble in the recordings. I feel like I'm getting close to belt-drive clarity while retaining the idler slam. This is confirmed by my hi-fi test record.
All in all, I am happy with my purchase. I was skeptical but the improvements are clear, and I'm getting more out of a great cartridge while satisfactorily addressing some of the weaknesses of my turntable.