A simple solution to injecting CO2 without reducing flow is just putting one of these pre-filter, injecting CO2 directly, and then using the filter as a diffuser. Obviously you need a non-return valve just before the T-piece to stop aquarium water going into your CO2 canister.
Works great for me: crystal clear water, CO2 levels high, no flow reduction and no variation in CO2 rates you get as the UP diffusers get gradually blocked up. Never had a problem with the filter either. That way you can keep injection totally stable.