Not very much. I drilled holes in the paddles of a couple of powerhead rotors, and didn't see a visible reduction in flow. This kind of pump just swirls the water so centrifugal force slings it out the outlet port. Nothing has to fit closely to make this work well. A lot of the "needlewheel" modifications to powerheads leave you with what would appear to be a non-functional rotor, but they still pump water.
I was thinking of putting together a sump and adding the co2 to the sump pump. That is why I was asking about the loss of flow and if I needed a bigger pump.
I drilled lots of holes in the rotor paddles on a small powerhead, and now I use the same powerhead to pump water from a bucket into the aquarium. I still get what looks like the same flow. I was very surprised when I saw this. Powerheads produce almost no head pressure in any case, so you can't lose head pressure by modifying the rotor.
I think the best use of a powerhead with CO2 is to run the CO2 bubbles through the powerhead inlet, so the rotor chops them into fine bubbles and then blows them around the tank. If you modify the rotor by cutting each paddle into a couple of paddles, as Tom Barr does, you get even better chopping of the CO2 bubbles. You still get the water circulation benefits from the powerhead too.
That is what I do in one of my tanks and I think it works very well.