I’ve seen a few people on the interwebs claiming sound improvements
It depends on the particular problem you're trying to solve, and
if you have a problem. The problem is usually
noise so don't give any credibility to anybody who says it improved the "soundstage" or made the sound "more clear" or any of that vague-unmeasurable nonsense. Short dropouts or data corruption will sound like noise (a "click" or "pop") or with longer dropouts you'll actually hear the loss of audio. It turns-out that jitter sounds like noise too, but the only way they know that is by artificially introducing jitter until it's orders-of-magnitude worse than you normally get, and then a defect becomes audible. It's impossible to "accidently" introduce frequency response variations or distortion.
I don’t expect my iPad to add any unwanted digital noise in the chain, so would anyone with a similar setup as mine ever need such a gizmo?
Usually the digital is fine... i.e. You rarely get a corrupted file when copying to/from a thumb drive, etc.... Real-time audio is a little different because it's time-sensitive, but if you' are getting glitches in the digital audio it's usually "something else", not the USB
connection.
But, noise can get into the analog electronics through USB power. Computer power supplies are notoriously noisy because of all the digital switching going-on inside the computer. It's not a problem for the "intended purpose" because digital data is highly immune noise. But analog electronics (and our ears) are highly-sensitive to noise getting-into the analog-side of an ADC or DAC. It's most common with USP powered audio interfaces where noise gets amplified by the microphone preamp. It sometimes happens with a DAC or USB soundcard but it's less common since the DAC generates line-level signals with no amplification. It's not an issue if the interface has a separate power supply.
If a gizmo has a truly-isolated the ground it needs a separate power supply, or a battery, or a built-in DC-DC converter. With a DC-DC converter the power is being re-generated so it can be cleaned-up. I assume the Topping device has a DC-DC converter so it's probably cleaning-up the 5V power as well as eliminating any ground loop problems.
