Q: I'm dealing with a concrete ceiling in a condominium. If I use LED recessed lighting, do I still have to drop a 6-inch soffit? Someone told me that LED cans take up less space than traditional incandescent cans.
A: Your main challenge here is the concrete ceiling. What the contractors usually do is drop a ceiling or soffit in the kitchen or a hallway in order to provide space for recessed fixtures. Then they throw a couple of track runs for the rest of the space. Yes, there are shallower LED recessed fixtures, but they don't have to be LED. Low voltage fixtures with remote transformers can also be quite shallow, and they can also accept an LED MR16.
Here's a thought: As an alternative to building soffits, you might consider installing floating ceiling panels. There are companies such as Armstrong that make modular suspended wood ceilings. Or you could make them from painted modified density fiberboard (MDF) or have an applied wood veneer. It would be best if these panels could be located such that they float over an existing junction box. Otherwise, you'll need to do some surface wiring in order to get the power where you need it.
If you go this route, you can also install a run of perimeter dimmable LED lighting. Then you are not only providing accent light from the recessed fixtures, but also a lovely layer of ambient light for the room from the indirect lighting flowing out from above the suspended panels. Please forgive me for going way beyond a simple answer to your question. I'm on a double latte high.