You can't cheat physics. If you dump X gal of water from Y height it will have Z kinetic energy minus efficiency losses. We've greatly improved the efficiency of the water dumping mechanism over the years but a high end low flow toilet of today is still nothing compared to a high end toilet of decades past. That said, there's a lot of crappy old toilets out there that don't flush any better than a modern low flow toilet and take 5+gal to do it.

Source: Moved into a house that was once owned by wealthy people. The upstairs toilet was clearly top of the line circa 1940s and is awesome. The downstairs toilet was clearly subject to some serious compromises for packaging reasons and is nothing special.