You could borrow an LS28 from a dealer to compare. But my guess is that it would not be a huge jump. I would say that moving to a used Ref 5se would be a much bigger jump.
A few years ago I had the chance to buy a dealer demo LS27. I passed because I did not think it was a worthwhile upgrade from what I already had. For fun I borrowed a Ref 5se to try in my system. Tuesday morning came and I was suppose to take the demo back. I called the dealer (who I had known for years) and told him I did not want to return the Ref 5se. I would much rather buy his demo unit. It was so good I did not want to part with it.
I am still extremely happy with my the Ref 5se, So much so that I bought a Ref Phono 2se to pair with it. I have no desire to upgrade them. BTW, I have own ARC gear since the late 1980's when I bought a D70 and SP8. So I have own a few pieces.