I can't fully understand.
You are using a 25k pixel width texture to obtain a 3k pixel width image. Ok, the wall seems real, or, better, is real....I mean, it's a flat picture, I think you can have same result using your 25k picture as a layer in photoshop behind the scene (resizing it...). What is the reason why you make Corona to reduce your picture size instead of reduce it with photoshop and then use a coherent size bitmap for your render? The only reason I can find is that you need a 25K render, do you? Or is just a test? I'm also curious to see a non-flat view (I mean not perpendicular view) with your 25k texture, to see how it works with bump or displace or gloss etc. etc...
BTW, it's just to talk about it, and to learn together.
Alessandro