Hard to say since I've only driven the 3.73s. I really can't imagine the difference to the 4.10s is that drastic. Sure, it's better but how much? If I were to do a gear swap it would for sure be to the 4.56. As a side note my dealer recently told me this wasn't possible but I'm thinking he didn't know what he was talking about since I've read of others with the same truck doing the swap with no problem.
Anyway, as you can see from reading the posts, right around 10K is the time to start considering the diesel. The whole 5th wheel thing is a quirk though. The diesel of course adds pulling power but reduces payload by 750# or so due to the added weight of the engine.
I fully agree with many that the 10,000# GVWR rating on a 2500 is largely a numbers game to keep these trucks in a certain class. I generally have no problem getting a set of airbags or helper springs and assume I have a 3500 truck. It's just a big disheartening to spend +/- 50K on a truck knowing I'm going the technically overload it. The other option is to get a 3500 and live with super bumpy leaf springs. After driving the 2500 I can tell you the coils are plenty stiff for daily driving and I don't imagine the leafs are any softer.
I'm currently in the process of getting airbags installed to help my coils and will run with my setup through the summer and likely beyond. A Cummins well may be in my future but I've just got too much other stuff going on right now to deal with swapping trucks (and 5th hitches, etc.). Overall, I'm towing at the max of the truck and it does fine 95% of the time. The other 5% is decent and doesn't justify and immediate need to change.
On Edit - one last thought OP, you keep mentioning 9500#. Keep in mind that's the dry weight. You're going to be at least 1000# over that with barely anything in the trailer. Likely closer to 2000#. I have just basic blankets and camping stuff and I gained 1500#. I never weighed my trailer empty so I have to trust the yellow sticker. It sure doesn't seem like I put 1500# in it.