The best example I can give you is what happened with the Mustang. Saleen took a stock Mustang and improved the look, aerodynamics, suspension, under the hood performance, improved and revamped interior, and finally "eye candy paint jobs". So much so that if you see a Stock Mustang and a Saleen Mustang side by side there is no comparison. That is what CM7 is for Android: Improved performance, optimized, and added features. Software releases in any field will always improve their product with updates, patches, fixes and improvements: whether from the OEM or from aftermarket sources. I have not delved into custom ROMS, but this is what will make me jump in.
That comparison doesn't really hold up though because you are using a stock Mustang vs. a Saleen Mustang. I am not talking about Stock GB vs. CM7, I am talking about ApeX (for instance) vs. CM7.
That would be more like a Saleen Mustang vs. a Mustang Cobra if you want to use your comparison terms.