I get the full 13.7v with a 1 wire. That's measured with a gauge wired to the fuse panel under the dash. Even at the fuel pump, toward the back of the bed, it's above 13 volts.
Not all 1 wires are the same. I have two, one is a Delco, converted to 1-wire. The other is a Powermaster, but it appears to be their own design, not a Delco logo anywhere. The delco needs to be rev'd to about 1500rpm to start charging. I never have to rev the engine for the Powermaster. My engine idles a little higher than a stock engine, so I'm not saying you absolutely won't have to rev yours, but it is much better about that than the Delco. Both provide full voltage.
Another thing at play is pulley diameter. Too large a pulley and the alternator will turn too slowly. If the vehicle is equipped with a 1/2" belt, then the alternator pulley is too big as it is impossible to get it small enough for a 2.5-3:1 ratio with the crank pulley. 3:1 is best for a stock engine or an engine with a near stock idle speed. Of course, the pulley should only have a significant effect at idle.
Perhaps in the future, when I have more electrical accessories, the one wire may not cut it. But so far, in well over 10 years of daily, year round, all weather driving, it has been flawless for the simple, mostly stock wiring.
Lawrenceville, GA