Is your filter outlet adjustable? If it is, can you change it so that it's not pointing at the surface? Reason being is that if you can point it elsewhere, the CO2 mist will be pushed back down into the water and will therefore remain in the water for longer.
My diffuser sits under the spray bar and I get a nice up and over effect whereby the bubbles go up, reach the flow from the spraybar and get pushed back down into the water before dissipating the usual way.