The SJ 28 looks like a fairly roomy, pretty typical boat of her era... While maybe not strictly an IOR influenced design the rig is similar to those boats as they were being done in the day.
She's probably on par with a Ranger 28 (though not as handsome) or other similar boats from that time, with the same limitations that the 'skinny main big headail' rigs "enjoyed"... That article in the site you linked to is pretty thorough and makes a good read.
Unlike their 24 and 30, the 28 is an in-house Clark design, not a 'name' design (eg Bruce Kirby) however they did some nice boats.
I remember racing against one in the 80s.. it did pretty well.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)