The Vreg is supposed to back off the alternator output when the battery voltage reaches 14.5 V. But, the Vreg sees (1.85 + .14) = 1.99 volts less that what battery voltage really is. You read 15.5V minus the 2 V distribution loss = 13.5V at the Vreg, so the vreg thinks that battery needs full alternator power even when it is, in fact, fully charged. The Vreg will try to charge the battery until it senses near 14.5V on it's sense inputs. At about 15V applied and fully charged, the battery switches from chemical conversion (charging) to electrolysis of its electrolyte, dismantling the Hydrogen and the Oxygen from the H2SO4 electrolyte solution (it looks like boiling).
Also the 150 Watt alternator spec is a minimum spec. No two alternators are exactly alike. As humans cannot make perfect devices, some can make more power others. Perhaps your is more capable than minimum under good conditions. In this circumstance it was actually getting 13.5V on its alternator field, allowing it to effectively go into battery overcharge runaway. And, would continue to do so as long as the RPMs were kept high. After electric start and idle operation the battery voltage will come down lower and lower. You could probably drive your bike a good long while, if you kept the RPM high, started out with a fully charged battery. But, once the battery gets into deficit conditions, it will have a hard time recovering on its own and spiral downwards. The stock bike needs about a 6 to 1 ratio of over 2500 RPM to idle rpm operation to keep the battery topped up. With your power distribution lying to the Vreg, it simply can't do it's proper job, and tweaking the vreg to attempt compensation is exactly the wrong thing to do. I'd bet once you correct the V loss, your headlight will be brighter and blinkers work better, too.
There isn't much about the electrical system that is static. There are multiple conditions and variations while it is operating. Because the voltage does vary with battery charge level, the power draw varies with the voltage. According to ohm's law, a constant resistance draws current in relation to the voltage provided. I = E/R.
Your voltage runaway observation was a corner case scenario. You've already encountered the down side of of the battery charge issue. But, the stock bike wasn't made to sit idle and/or hold idle RPM for far more time than above 2500.
30 amp fuses.
He put 30 amp fuses in the bike because he kept melting (not blowing) the main fuse, likely exacerbated by the 3 ohm coils. 30 amp fuses take more heat to part their element. Normally you can hold your thumb on the proper 15A fuse and feel it be warm. When the fuse clips get oxidized and you raise the current passing through it nearer to its melt point, you'll get blisters on your thumb. You can melt fuses with a blow torch or soldering iron too, no current required at all.
If you want to know how fuses do their job, I can explain that, too. But, I think you have enough to do now correcting the V loss issue.
Cheers,
