Another vote for going to the ceiling. I have four inches of "library" molding, and four inches of crown molding on top of my cabinets. I certainly didn't want that much molding, but needed to hide the vent hood exhaust tube that went above the cabinets to the outside, and this seemed to be the easiest and least obtrusive way to do it. Now that it's done and I've lived with it for awhile, I really like the finished look, as well as the architectural interest it provides Here is a link that might be useful: kitchen pic's

While in pics my molding appears to be similar to the walls, they're actually very different in person. The walls have a significant amount of texture and color in them and the wood is of course smooth and shiny and different in tone. I also have the opposite in my victorian where the crown molding is painted but the doors and trim are not. Again, no racing stripes. Now in that last kitchen....we don't see the kitchen well. Personally it does get that racing thing visually from a computer because it appears that the chair rail, base board and crown are all the same, with no differentation in the wallcovering or color between, hence the stripe. Now closer up we might find that the lower area has a texture that defines it....which might make it work nicely in person, but on the computer, it does look terrible. But that's the chair rail. Remove it from the pic and you have no race stripes. Boring room maybe :P But no stripes ;) IMO Painting that molding would do nothing to effect the stripe issue. Painting the walls (lower) a different color to accentuate the molding would solve the issue immediately :) But it's again a different issue since the OP does not have a chair rail problem. I have several rooms with different molding colors on the window verses the crown, as well as rooms where the crown and the toe don't match. They look lovely. No stripes :) just nice finishes designed to work within the room they're in. The reason the house flows well though is because I kept the main trim (windows) the same color. In this case the OP can do either and either will look nice. Because the mantle build out is the same color as the molding it would flow well together, but given it's also a specific item (mantel) it can have that furniture look by painting the other molding and leaving it the wood tone on the actual mantel portion. Mantel? Mantle? Manaties? gad I'm a spelling idiot!

A 4 1/2" profile on crown would give a drop of 3 1/4" - 3 1/2" which, IMO, would look skimpy with 9' ceilings. What your carpenter did by adding a backer board with a profile is frequently done in our area (and appears to be used frequently elsewhere given the number of photos on Houzz of this treatment). If the size appears to be proportionate when looking at it in person, then the product looks fine once it is caulked and painted. If the total affect still looks small to you, I would stop now, return what you can and go with a larger crown. You can do a search here on Houzz for crown moldings in photos and you can see the finished product.

I'm surprised the installer didn't address that during installation. I would have used an extra piece of trim, in the same color and flat, and installed it behind the crown molding. Thus, there would be a flat white area where the gap is. Yes, it would still create a visible line, but much less visible than a shadow or gap. Since the crown is already installed, caulk.