It will still use the same amout of menory when scaled. This is because the texture is loaded then the rendering scales it.

Each side uses a shared copy of the texture.

As far as I know the texture is still loaded so there is no reduction in memory needed. It may however not load it to video memory if there is no other visible side that uses that texure.

As for the FPS, I have know idea why it would drop at all. It should stay about the same. The only reason I can think of would be that it is holding writing to the buffer in windowed mode so as to match the refresh rate of the windows screen thereby faking a refresh so that the A6 would go on. In full screen mode it would match the refresh rate of the video mode.