Flush valves aren't usually sold on their "flushability", balloney. The modern ones, particularly, all seem to be even better at it than the old diaphragm type.
Something I came across recently might be of interest. I replaced a f/valve, and the power was reduced considerably....... enough for me to take the cistern off again for a good look.
After some head scratching, I realised that the spigot end of the valve (the threaded end that is inserted into the rubber "doughnut") was way too long. Being only a few millimetres from the china. The water just didn't have enough room to flow properly.
I simply cut some off the end of the spigot, and full power was resumed.
Might be worth a look.