I'm sorry that this doesn't really answer your question, but I have a thought that may be helpful.
Let me preface this that you are preaching to the choir when you talk about non-performing-but-beautiful-Kohler toilets. I had a lovely Portrait that was the worst-flushing toilet on the planet. And I have a nice-looking AS Galleria lowboy in the city that came close, until with the knowledge I was inspired to learn from this forum I got it working as best it can. Now it's merely bad.
So, back to the topic. You want a dual-flush because you are environmentally-conscious, I would assume (or maybe you are just trying to save a little on your water bill.) Because a dual-flush averages the water used between 1.6 and .8, it is considered to be generally in the same water-saving arena as a full-time 1.28. Both are considered Watersense toilets, and both are eligible for rebates where such rebates are offered. For what my opinion is worth, you are likely to be as good to the environment by having a 1.28 gpf toilet as you are having a dual-flush.
And the 1.28 comes with other advantages. Because the mechanisms for dual-flush are necessarily more complicated than the basic flapper flush used on all other Toto gravity toilets, you are looking at parts that are just plain more likely to break, and more likely to be more expensive and less-readily-available when they do. And the flush on most dual-flush toilets -- almost always a washdown flush -- is going to give you a different experience than a G-Max or a Double-Cyclone flush, the latter of which is really the best out there as far as bowl wash while still clearing the bowl. (I have the Double Cyclone in our Carlyle II.)
So, my suggestion to you is that if you like the Toto Guinevere, get it. You won't be disappointed with its performance -- in fact you will likely love it -- and it's a Watersense-appproved 1.28gpf Double-Cyclone toilet that comes with Sanagloss and Universal Height standard. And it's good for the environment.
I don't know the AS toilet personally. But I do know (and you must from reading the forum) that Terry has found the quality control at AS to be appalling. And if you look at the reviews by actual customers on the Consumer Reports web site, half of them have comments about how the toilet they got was unglazed in spots, wobbly, not formed correctly, etc. It confirmed that what Terry reports seeing in the field is what customers are experiencing with their products. I guess AS just assumes that the customers won't uninstall and return the toilet, so it sells it even if it is plainly defective. Sad.
Hope that this maybe helps.