It is official. I talked to Johannes and was saving him some time with everything else he has to do writing it here. Trust me, it is to all of our benefit otherwise with 22+ cars on the grid with double size textures, it is pretty much 4 times the texture memory.
As for how to do it. You can just re-open just the .dds files, scale them down and save them back out in the proper formats as mentioned in the post. Some are DXT-5 with Alpha, some are DXT-1 without.
As far as the templates having some locked layers in Photoshop, that doesn't matter for scaling -they will remain locked unless you select that layer and unclick the lock icon manually. In those cases, you dont want to scale each layer, you want to scale the whole image. Go to Image --> Image Size and match the size to the corresponding texture in the example folders (EX) for each of the chassis types. Some of the textures are not square (i.e. the helmets, but just keep 'constrain proportions' on when you are re-sizing, so that the texture stays proportional and make sure that the largest scale value is 1024). In most cases if you are using templates, you will be changing 2048 to 1024.
As noted above, the difference should mostly be seeing file sizes change from around 5,462 KB to 1,366 KB.