I'd go read up on the MaP testing results at the CA Urban Water Conservation Council site. They've been continually testing new models since 2003. The appendices are a quicker download if you don't want to read the whole report.

Here is a link that might be useful: CUWCC MaP Testing

Thull:

Thanks. I did check the MAP out. Unfortunatley, they have not yet tested the WS 1.28 gallon I was looking for because the testing results were left blank beside the specific models I was interested in. Hopefully, they are in process of testing them.

There is very little real world difference between the 1.6 and the 1.28
With the 1.28, they tend to shut off just as the bowl fills. Some of the older toilets would run on for a bit after the flush, "overfilling" the bowl.
So......performance doesn't really change much.
I both in my home, and they just work. And customers are pretty much saying the same thing, both in the home, and they both work.
Performance is getting better and better.
Eco Drake elongated bowl 1.28
Ultramax with Sanagloss 1.60
Ultramax II with Sanagloss 1.28
Terry Love