Here is my 20 minutes homemade flowbench :
The idea is to time the exit of the water through one of the exhaust valves (they have their dedicated port) and compare the result between the modded port and the stock port.
So far I have to improve the leaks between the "cylinder" and the head and to make sure the gasket (destinated to seal between a WC and its tank !) has no adverse influence on flow or at least evenly.
To make sure the result is mesurable, I chose to lift the valve only a small amount (I assume half the lift)