We did have the arch added to our 258 and we used it for wakeboarding. You are right that its really not a ski boat but it made a great wake and our kids loved it. Our arch was built with the boat and from what I was told, they reinforced the body at the factory to help with the stress.
If you have the solid arch, I would be concerned how structurally sound that would be for towing. We needed to fold our bimini under the arch to ensure the tow rope did not catch. I think you would need to rig it going forward rather than back. We used bungee cords to hold the bimini.
The wake was a little rough but at about 22 mph, my son could jump from edge to edge. If I can only convince him to try it with our 338!
Just my 2 cents.