Thing is that it's not the number of polys causing the slowdown. It's the number of textures. Using the alpha would mean bigger (or same sized - I forget which it is) textures and thus wouldn't cause any speed up at all.
Same Size, DXT5 are 32 Bit Alpha Textures, it saves the Alpha even if you don't want that channel, it should be the same size and memory usage. For that there are DXT1 and DXT3 textures.
I know the Textures are the one exausting the RAM, but DDS use MipMaps, while the Model of the 3D Shockwaves are just Detail-0, that means 804 polys for every Shockwave even at unseen distances. I am sure that in INF (and others), as an example we get up to 6 shockwaves for ship simultaneus and with a Wing of 6 Ship, we get 6 x 6 = 36 simulateous Shockwaves, with just LOD0 it is just 28944 polys, that's only per 1 Wing of Bomber if there were even more Wings (which happens), , that's a good number compared to FS2 Campaing...
I don't really know how Alpha is managed by SCP, because I can't get any Alpha to work for Texture Maps, not even Binary Alpha. For Backgrounds it works totally OK, as it should be, but for Models it works different, I can't really figure how....
Never mind that if you're going planar you really should be falling back to 2d anyway. There is no instance where a 2-polygon shockwave will EVER look better than the bitmap ones.
This simple Model just has 12 polys.
A Model rotates and has an unfixed position for Shockwaves, a Bitmap is Static it doesn't rotate or anything, the viewpoint is always the same. The difference is clear...it seems that EVER just means EVER when your affirmation is not wrong (2D Earth Background have frequently more quality and colours than a 3D Model)
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The Main case, is this, a Square Planar model can't be seen from Left / Right / Botton or Top view, it is planar, the problem is that the Shockwaves made by DaBrain doesn't have a real texture from those sides, so at the moment it would be the same, I mean, have sometime a Shockwave hit you from one of those points ?, you should see two lines just 2 lines, it happens with the actual Model of Dabrain, it's is rare and not so...in exchange of see that we should see a Explosion (Blue, etc.) wall coming over to us, but at the moment that can't be made, with a Planar square model that would be totally 100% impossible to be made.
So, actually both could have the same graphical quality, I don't really see too much difference