A normal battery on full charge is 13.8 volts, and while charging can be over 14.5.
The reason stock #16 wires works for some people is at 15 amps the voltage drop in 10 feet is about 0.6 volts. With a fully charged 12V battery on a running alternator you'll be at least 13.2 volts.
If you change to #10 (same 10ft length) you'll be at 0.15 volts drop, so you pick up 0.45 volts.
A good fuel system has way more headroom than that.
The ground wire length to the chassis also adds to the drop, as does the fuse and the relay contacts and especially any push-on connections. But as a general rule as long as you are not on the ragged edge of pump pressure or volume, you are OK at 15A in a car with an alternator with 16 ga with voltage.
My bigger worry with 16ga would be wire heat with minor overloads. A fuse does not blow exactly at the fuse current, it can sometimes carry 2 times or more the fuse rating for a minute or more depending on temperature and the fuse type, and any holder resistance at the fuse contacts. The OEM relay is also marginal.
I am helping a neighbor clean up the mess in his car and his fuel pump has an unnecessary 5-6ft long ground lead. It could have been a foot long.
Because of the heat and safety, and because the OEM relay is out of headroom on the contacts and probably old, I'd do like someone suggested and just pull a 10 or 12 wire and use a new 30-40A relay either driven from the old system some way. But I just wanted to point out why some people have no problems. It isn't really the voltage drop that is normally the real issue, it is the head room and safety.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)