You will note on the toilet report, when clicking on the individual hyperlinks, that many of these toilets have glazed trapways. Among these are the Western, Toto, Kohler. The Gerber Ultraflush is unglazed, however, not a problem!
Most trapways are less than two inches. Some are a little larger than two inches. When trapways get too large, it is hard to create the siphon for a good flush.
I am not familiar with a Gerber K-11 so I am no help there.
There really is no government mandate on how well a toilet flushes, only that it doesn't use too much water. Builders are stuck with the unenviable position of building homes competitively to stay in business and producing a home "good enough" for the average homeowner. Many are working on a 5% profit margin to stay alive. You can see that a "little upgrade here and there can create problems for the builder. Showing your builder a copy of "Terry Love's report on low flow toilets" may help him to "spec" in toilets that work better in the future. Because you are having problems with the current toilets, I would suggest toilets at the top half of the report. Not all peoples habits are the same, and you may want to consider "upgrading" to something with a stronger flush.