If I could place the RX100 sensor in A500 for bird shooting, .........

Wrong.

Suppose I'm out and shooting with two Sony cameras, an A900 and an A100. Suppose I'm using the aforementioned 100-300/5.6 Minolta. Suppose now I mount it on the A900, and shoot at ISO200 1/800 f/5.6 to get a 'proper' exposure. Suppose now I mount it on the A100, set it at ISO200 and f/5.6; what should my shutter speed be to get an exposure as similar as possible to that from the A900?

(hint: it's not 1/300).

That's what people mean by "light gathering", that's what it has meant for *decades*, and while I do understand why you got to the conclusion you did, for most people and most situations it's an entirely useless calculation to make. Photographers do not count photons.

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