Even with mip-mapping, the large version of the texture would still be in video memory though, right? It's true that it wouldn't contribute as much to fill rate, but it would still be a load on VRAM/the GPU bus. So, while the previous answer is correct, it's not the whole story.
Bottom line: profile your game, figure out where your bottleneck is. Mip-mapping certianly won't hurt.
AFAIK the only billboarding in Unity is trees that are integrated into the Terrain system, though that wouldn't stop you from rolling your own billboarding system (if you have Unity Pro, of course. Because render-to-texture is a Pro feature.)
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