I've read a few articles on horn loadings, and I agree: too much work and a little silly for most applications. But using a proper driver in a horn loading can get loud for sure. I'm not an expert... but I thought most horn loadings were tapered though. That design kind of has a bastardized transmission line / horn / wtf look going on.
Maybe worth doing on the educational (or shits and grins) level... having the loudest 8" on the block is bragging rights I guess //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif. Personally, I'd play with a 6th order bandpass box to get loud on a smaller driver experiment.
***edit** Looked up tapped horn design... something I was not familiar with!