As beautiful as QS Sycamore is. I don't think a whole box or much of any project could stand all QS Sycamore. It's really better left for smaller pieces with another wood as the prime. A QS Sycamore coffee table for instance would be overwhelming, but a drawer front on a walnut or cherry coffee table would work.
You are correct ammonia fuming is just to darken the wood it is not a protection finish. The piece below was fumed then hit with a coat of garnet shellac before having a film finish applied for protection, in this case waterlox
Here are samples of what fuming did to this white oak. The bottom is fuming at 0, 2, 4, & 6 hour intervals under the tent. The middle is fuming with one coat of garnet shellac and the top is fuming, 1 coat garnet shellac, and 3 coats of Waterlox