Jim, check a report on low flow toilets at
formatting link
Go down to where you see a link for "Final Report." It's about 30 or so pages. Look for the sections on flapper (a/k/a flush valve) replacements, after market flapper replacements, or standard flapper flush volumes.
You will also find a host of information on the various performances of the low flow toilets. There apparently have been improvements in the last couple of years, which may account for some of the experience differences people are having with supposedly the same toilets. Kohler Wellworth, for example, has numerous past models.
You can get one that allows more water after replacement with a standard flapper, but even then, you may not get the same performance as simply buying one of the highest rated toilets that uses only the
1.6. But you may. Who knows? I am facing the same dilemma and am doing much research. It's not like buying a TV, where you can easily return it if it doesn't work right.