The really serious flaws in the forumla are because it hasn't been updated to account for changes in SL. It completely ignores normal and specular maps, it does not take into account that texturing tend to be far more heavy than it used to be
They should have made the ARC increase from textures directly scale with the pixel count. Right now the formula is an absolute joke.
256 + (16 * ( ( height / 128) + (width / 128) ) )
So, a 16x16 texture adds 260 rendering cost.
A 1024x1024 adds...512.
It's as bad as the LOD mistake, if not worse given the average content. What they essentially should have done, is make it a simple formula for the dimension weight like (width * height) / 1024.0. That would result in a 1024x1024 being 1024.0 render weight, while a 16x16 is merely 0.25.
On top of that, yes, add a fixed amount per texture to that - to account for sheer amount of textures used. They already did that with the '256' per texture - just the added scaling calculation is worthless.