Yes and / or maybe.
Your stuck thinking about the value of synergies, but synergies don't always happen just because forces are joined together. Each, at this time, is a different tool for a different job. Consider it a Sum-Of-Parts valuation.
While the NRA does have a grassroots element, it may also be the most powerful lobbying organization in the country. I believe this can both help and hinder the causes they throw their hat in. Their focus is more national in scope.
While OC, is clearly grassroots, and may appeal to that cause (most of the time it is a state by state issue) far more effectively. I believe it might be better for the OC movement not to have the anti-gun types stating that OC is some puppet of the NRA movement.
In addition (and this is just theoretical), the OC movement may not appeal to all NRA supporters (the NRA would not gain as much as they lost) & the NRA may not appeal to all in the OC movement (OC movement may not gain as much as they lost).
They report on it often enough. As an organization, they have some limit to their resources, and are focused on goals of the majority of their members (they do use those surveys they send out).