Here is a link
off a SJ28 page I found when looking for my current boat. Thought about the SJ28. That 14 page PDF is a history of the Clark Boat company that manufactured SJ's and C-larks among others.
It has a part where as Cobra Charlie mentions in your other thread, ie the Venture starting the trailer sailer era. It has a few other notes about SJ's as to why they might be a bit better than other brands and a bit quicker too. On the other hand, it mentions that there are about 6-10 SJ21's that were made a layer or two thicker than most! IE they went into the chater trade in Fl, and they were knowingly going to get beached, and removed via a long forklift, hence the thicker hulls, along with one heck of a lot slower than the normal thickness hulls........
You can race PHRF with ANY boat. There is some portions of racing that is nice to do 1D too. IE sharing sails, rigging and other parts, idea's etc if need be with in the club. With in this thought, you may also learn to sail a bit quicker if you have the same boat as others within the club, as there will be more how to make the boat quicker etc.
Also, I am recalling, while the Mk II version does race side by side with the original deck version, I seem to recall that the original cabin version is a bit quicker generally speaking. I did not sail both models, so will not speak from experience on this item. This may be spelled out in the article I linked too!
Good luck with choice, in realty, there is no WRONG choice. Other than my choice! oh yeah, I did not have a choice now did I!!!