The Delta you have will never even get close to the "rated" 1200 CFM as that's likely a free-air figure. Mass market DCs are like that. The CFM numbers you see from worthy cyclone manufacturers usually are accompanied by a fan curve which will tell you how they perform in somewhat real-world situations. Best performance is going to come from long-cone cyclones such as ClearVue and Oneida. Short-cone designs like mass marketers Grizzly, Jet, Laguna, JDS, etc., do not provide nearly as good separation based on most sources that are knowledgable about such things.