If you use more than one of the same type of USB gamepad, then they will randomly switch positions with every reboot, making all that work you did a big waste of time.
I think it might be okay, what I am looking at is individual desktop control panels. Like a bunch of specialized X-Arcade solos. So one panel would be plugged in all the time, the others as needed. I think MAME will recognize them in the order they were plugged in, which would be fine. (At least I hope it will, if it treats joysticks the same as USB mice, I think I'm okay). This sounds like a question for UrebelScum, though!!!
I have never seen a fully "wired" joystick with more than 8 inputs total. An original Sidewinder will get you 14 though (don't bother with a USB sidewinder, as they are hard to hack).
After I posted this I thought I might need to check 1Up's dual strike hack again, even though DS's are discontinued.
Once you are hacking more than two gamepads it just becomes cheaper to buy an I=Pac anyway.
Maybe not, I want 5 panels: two X-arcade solo styles, a standard X-Arcade with rotary joys (requires two pads), and two Assault style panels for vindicators. So 6 game pads total. I would like them all USB so I can connect and disconnect as needed. That many inputs would otherwise need an MK64, which I have worked out how to do, but then I need a bunch of DB25 cables, and an enclosure for the MK64, so figure $100 total for encoders and wire to go this route, so if I can come away for $16 per game pad, I break even, and if it will do more what I want, then if it ends up being $30 more than that ($20 per game pad) it's worth it.