Can't answer what other mfg.s do, but I don't think you overbuilt your deck, at least not to any significant degree. Full of water, an Optima weighs almost 5000 pounds, then allow for up to another 1000 pounds if you load it up with 6 or 7 people, and you are spreading out that weight over the roughly 65 square feet of deck that the spa sits on. When it comes to stuff like this, you can't go wrong overbuilding.
You never know what the future may bring- someday maybe you'll want to install that huge Maxxus, or something like that- if so you'll be set to go.
You made a wise choice, I think, to build your deck to handle that load.