Recently I converted all (14) of the toilets in a local Bed and Breakfast to the One2flush dual flush converter. The results were as I knew they would be. Every toilet that I converted operated well in both the half flush and the full flush. The types of toilets that were converted were American Standard 1.6, Kohler 1.6, Eljer 1.6, Eljer 3.5 and Mansfield 3.5. In all cases the half (or short flush) had a decent swirl and bowl evacuation with less than 1 gallon in the 1.6 models and 1 to 1.2 gallons in the 3.5 models.
There were two toilets that I chose not to convert. First was a Hilo HMAS wall mount from the early 20th century (8' tall) and the second was a 1950's Standard modified to work as a Hi Lo type toilet. The tank was elevated to about 7'. At first I thought this would be a good example of how even old units like this would work well with a dual flush conversion. I proceeded to draw up plans for a modified twin chain handle that would compliment the aesthetics of the two different bath rooms. Next I went to look inside the tanks to see if I would have to do any special internal modifications to get the One2flush to work. I found that the only modification I would have to make was to adapt the 1 1/2" outlet on the Hi Lo to a 2" to work with the One2flush. Easy enough to do. Then I looked at the reality of the situation. There was only a little over 1.5 gallons of water in the tank. I test flushed both of them and saw that they both operated perfectly and probably better than most new dual flush toilets. I was in a quandary as to whether or not to try and improve on some 100 year old technology that worked so good. I opted to not convert the Hi Lo toilets, because it would be a waste of time and energy and would probably not improve the function of such a good design.
My point here is gravity. And the notion that a six gallon American Standard won't work well with a One2flush. If the tank level on the American Standard is kept high, there will be enough water force to start the siphon action with a reduced amount of water. The best we have gotten on this type of toilet is 1.2 gallons on the half flush and the full flush at about 2.5 The only toilet that I have encountered where an over sized bowl is an issue was an old Universal Rundle where the tank was also very low. It didn't flush very good no matter what you did to it. I think the only solution for that toilet is a sledge hammer.