Agreed, the ignition module will be grounded to the engine block which is also the alternator's ground. That's as good a zero reference as you can get. (Yes I know there are some ECMs that control alternators that are grounded to firewalls.)
I once had to run a #10 ground wire from the alternator to the battery. That was to eliminate RFI (alternator whine). I know it created a ground loop bit it worked.
Some grounds from engine to frame are also to prevent aluminum erosion caused by galvanic reaction. Wide tinned braided straps work best. Ground straps from firewall to engine and hood are also best with tinned braided and can help with AM radio interference noise.
I get it...and obviously it was on one specific car. One cannot correlate that improvement to another car. Perhaps this specific car had a grounding problem?
A quick google search and I found a reference to the article...it was a Nissan 350Z and it was in the July 2003 Turbo magazine
Engine & Drivetrain - Finally: Ground Kit Dyno Results! - Hey all! I know a number of you have installed earthing kits, custom or otherwise. I also know it makes about zero sense to do before and after dynos because the dyno test costs more than the ground kit in most cases!! The nice folks at...
my350z.com
The original poster says 3.8hp gain, then the subsequent poster says it was a 4.8hp gain....
You can also buy it in rolls with eyelets every foot, 6 AWG equivalent. Often used in telco and data center grounding applications. But I don't see that particular product right now.
This 4 gauge ground cable is cut to length and is terminated with a copper terminal and lug. Copper wire strands are tinned for corrosion resistance and maximum conductivity.
Nissan and Infinitis from the 2000s were known to have less-than-optimal grounding and there are a lot of "kits" sold. They aren't for MPG or HP improvements though, but obscure things like slow/delayed shifts, especially when using the manual mode (either with the "tiptronic" type shifter or paddle shifters).