Some mic and preamp combos go together "synergistically" like peanut butter an jelly. Some don't. And some people like the sounds of certain combos, and others don't. So, I don't think it's appropriate to say going from a UA-5 to a Littlebox is inherently an upgrade, or not. It depends on the mics you are running and your tastes. It's an upgrade if you like the new flavor combo.
C4's > UA-5 is a proven combo, run many times by many people, and people keep running it because it works well together. I strongly encourage people to listen to lots of samples on archive.org of different mic/preamp combos and decide for themselves. If you can't find one with C4s > Littlebox then probably no one has tried it yet. It might be the greatest thing since sliced bread, but no one will know until someone tries it.