Yeah I too wondered why the author would target 60Hz at all. The fact that the SNES controller is polled at 60Hz is simply a consequence of the game reading the input at that rate. As mentioned in the other comment in this setup the polling of the controller is not in sync with the game reading the input at all. Thus even if you target a polling rate of 60Hz perfectly you'd actually have worse input latency than the original hardware.
It would be much better to target 120Hz or higher to reduce input latency and bring it as close to the original hardware as possible by ensuring there is always an up-to-date input state ready for the game to read.
I had a feeling the polling rate was probably to do with the frame rate of the SNES rather than some requirement of the controller. I'll try out a higher polling rate and see what happens.
Yeah I wouldn't be worried about the hardware at all. The only chips on there are two 8-bit shift registers and according to the datasheet at 5V they can run up to 3MHz. You're much more likely to run into the limits of the USB protocol than the SNES controller hardware.
If for the sake of argument we are plugging this into an SNES emulator running at 60Hz, then the stack between the USB gamepad and the emulator already has to handle that the gamepad is not rigidly synced to the emulated SNES and will presumably take every input from the gamepad and use it as the emulated input to the SNES next time it asks, unless a new one comes in first.
At slightly faster than 60Hz, the net effect will be that the delay between input and having an effect on the SNES will wander about 1/60th of a second over time, as the sync varies, and every once in a while a particular input will be overwritten before it makes it into the emulator. It may be very difficult to perceive this, though, due to other delays already built into such a set up. On a real SNES, it would be right on the edge of perception anyhow.
Being very close to the poll rate but not quite there is probably near the theoretically worst case. It would probably be much better to poll at 240+Hz, cutting the latency between input to a consistent 1/4ish of a frame. However, I doubt this improvement could be "felt" by very many people at all.
It would be much better to target 120Hz or higher to reduce input latency and bring it as close to the original hardware as possible by ensuring there is always an up-to-date input state ready for the game to read.