"Would he be better off with a ported cabinet, or is it possible to make a horn for this woofer also?"
a horn could probably be designed. ported would get you 126-127 db at the 2000 watts rated program power.
the problem is the driver handles 'only' 2000 watts program.
for that frequency range, he might take a look at the b&c 21sw152-4. it is a 21" driver with a huge 6" voice coil to dissipate heat. as a result it can handle 4000 watts program. ricci measured one in a sealed enclosure with bursts in the 136db range (rescaled to 1m).
in a ported enclosure, it has about 1db more sensitivity and with double power, you get another 3db, so it would have about a 4db edge on 5100.
4000 watts gets you 130db in model from 50-200hz.
"I have a couple 15" LMS-R's that need a home, and have thought about a beast similar to this. Any chance this could be slimmed down ~4" or so in width without much compromise?"
since the horn has four passes horizontally, reducing width by one foot reduces the horn length by four feet. that raises the corner up to around 25hz or so. i'd probably look for another design in that form factor vs. modifying this one.