Robinson Racing is legit.
It's not to say you can't get good stuff on Amazon, you absolutely can. I would stay away from the generic gears made from monkey metal.
I would stay away from Amazon generic gears, you get what you pay for. It won't hurt your car, but cheap gears break and then you have to buy more and replace them again, and again.
Don't over think this, that's my job. ;) It's easy to get caught up in the numbers, but the theoretical...