Often, the sites like to show a picture of a tiny OEM wire next to the newer larger, prettier grounding wire. That's great marketing, but what they never really show you is the equivalent power wires. I've looked at a few and generally, the power wire is the same size as the equivalent grounds. That's all you need, going to a ground wire that is bigger than required does exactly nothing. Grounding the ECU does nothing since it's all digital - it either works or it doesn't, your car won't run smoother because you put a bigger ground on your ECU. Not sure how you'd get faster shifts on the transmission from a better ground. TCU sends a signal to the VB to shift, maybe a bad ground could cause interference, but that's reaching.
Plus the kits are just supplemental, they don't seem to actually replace any ground wires, it's very indirect. If you ground your amplifier to the chassis, that's direct. You do it wrong, you get noise. Adding a grounding kit would be the equivalent of leaving the amp to chassis ground, then running a bigger wire from the chassis ground to another grounding point. How would that make any difference? I see claims about smoother injectors, but I don't see how? You're not touching the actual wires to the injector, you're just adding a bigger wire to their grounding point.
It's pretty easy to check how your grounds are. Get a multimeter and put one end on each side of the ground wire. If it's close to zero, it's fine. If not, your ground is bad. Try it with a grounding kit, I'd bet the numbers are the same. Strange how no company actually puts up a number showing their grounding kits have less resistance than OEM. That's the number to measure, not "smoother shifts" which you can't prove.