Pretty sure they're 5-bolt already. The thought about that torsion set-up wearing out had definitely occurred to me. As you know, the big idea behind it is that the rubber, after compression, doesn't want to spring back past it's original shape, like leaf springs do, making shocks unnecessary. But my thought, which you just verified, was that the rubber would tend to compress after flexing thousands and thousands of times and eventually it would probably stop wanting to "spring back" to it's original shape - hence the sag. With leaf springs one can replace springs or shocks. With torsion suspension I guess it becomes the entire axle eventually. Big improvement! ha ha!
I don't know how many miles are on this trailer or what was done to it prior to my purchase as I purchased it as-is from a private party in Broomfield CO last August. I have nothing to compare to in order to know if it's sagging down further than it should so I'll have to get it checked out next spring and see if it's worn out or what before I do anything to it. It doesn't seem like it's overly bouncy or anything during towing (I have an '06 Tacoma v6 4-door w/5' bed and towing pkg) and I don't use any sway control or weight distribution equipment and it seems pretty stable as long as I keep it to about 60-65 mph max, so it might still be ok.
This is my first trailer and my learning curve is on a very steep trajectory.