I hope no one takes any offence - absolutely none is intended.
I replaced the water pump when I did the HG job. I kept the old one....
I tried drilling the impeller - there's no difficulty what so ever. It's very soft. Judging by the ultra short chips it's gray cast iron.
I took a 10mm straight cutter and made a hole at the tip of a vane.
See photo. The last half a millimeter can be dremmelled off.... didn't want to touch the pump body.
Drilling the tips of half of the vanes would be a nice try for someone with a test setup..... or if someone comes up with a better "drilling plan"
Tapani