The NEC doesn't deal with the spacing of detectors. The information you're looking for is in NFPA72. I'll post a couple of pages (of course there's many considerations depending on the complexity of the job). Also (and I haven't been looking into what is actually out there), there may be some fancy beam-type, protectowire style, etc. detectors out there that could be installed at a lower height but again you'll have to dig around the internets a bit. In the PDF i'm attaching also see the different applications for different ceiling types (BTW what i'm posing is from 1999 fire code, and there's been changes, including the mess about eggshell/honeycomb style ceiling construction coverage, which they changed in the 2007 version but then the lab results allowing them to relax the rules were questioned so they reverted back - I never really followed it up all the way to see what happened or what changes will be made but again something you may want to look into.)
