Yes it would be faster. But you will still have to split the complete, huge, prerendered texture, into smaller textures.
Since the maximum "renderTexture" is dependant on your GPU,
i think 1024 would be ok for most gpus, but you will have to check it yourself.
Whatever, as I said you will still have to split it.
I will implement some kind of grouping into my tilemap so a 20x20 part of my levels tilemap will be rendered into one rendertexture.