I discovered a way to make the new PC toilets flush OK. Our new house had, of course, a couple of the PC toilets and neither of them would flush completely on one flush. One of them soon developed a leak around the flapper valve. I bought a repair kit at Ace hardware that replaces the flapper valve with a ring which glues onto the big hole in the bottom of the tank and a swinging arm gadget, a type of flapper, which seals the hole.
The gadget is adjustable for length of time it holds the valve open, and I found that by adjusting it to maximum amount of open time almost the entire tank will empty into the bowl. Since the new tanks hold almost as much water as the old ones, my toilets now flush as well as the old ones. I can't remember the name of the kit, but it comes in a tall green and white box.