I really was hoping, and I may have been a bit too vocal about it in the past perhaps, that scaling textures in Corona would be better at this point. As it is right now it is still a bit of a pain in my opinion.
The transform inside a layer option can work but its currently surprisingly buggy and I find it to be quite slow when scaling bigger images (4k+).
Project shader does work but it is cumbersome to work with because then you need to deal with the UV mapping on a per map basis and that can be ... A lot of steps sometimes not to mention weird-ish workflows. Once the nodal editor gets to a more stable point (<- I mean relatively bug free) it might be easier to work with because then you'll be able to instantly recognize when maps are driven by the projector shader without having to go into each and every channel etc etc...
While having it work flawlessly with the nodal editor and the nodal editor working reliably would go a long way at making this sort of thing easier to do I still wouldn't call it optimal because the project shader essentially overwrites your UV settings from your tag. So say you got a good ratio going between all the maps but you just want to make everything scale up using the material tag, well, that gets tricky and convoluted fast as you need to go into each projector shader and adjust things. Imagine having multiple project shaders layered in a layer shader in a couple of different material channels, oh the confusion :)
Personally, I do think Maxon should have resolved this by now and they kind of have with the new UV node in the C4D nodal editor but we aren't using that so I guess its a moo point. I do like the persistent nature of the Corona nodal editor and I find it a great addition to the toolset. So major kudos for that.
Now with that said, every other third party C4D renderer appears to have a solution for quickly scaling maps. Octane for example has a UVW transform node and it is so amazingly great and easy to use, sort of like how it all works in 3ds Max. Because each third party renderer tackled this on their own I think we should too. I would consider it quite a priority to be perfectly honest because scaling maps is something one does all the time :)
edit:
One thing that would also really be helpful for this often used operation is to have separate map previews in the viewport. Say you want to scale one of the bump maps differently, if you can have instant bitmap preview in the viewport that is incredibly helpful because IR can be slow in heavy scenes. 3ds Max has this solved natively by having a button that simply isolates that bitmap in the viewport and shows it properly mapped on a mesh. Since the Viewport in C4D is hard to access via the SDK I reckon this probably isn't a priority but I guess it would be nice to keep it mind for when it does get "open" :)