When I tested my headlamps for voltage drop I found nearly 3 volts lost in the wiring. The factory service manual claimed it was to be 14awg but it looked on the thin side of 16awg.
I made my own harness from 12AWG, Tapped the nearby alternator to battery feed wire to feed the relays, and now have about 0.3v drop.
I'd estimate 30%-40% brighter.
No more headlamp current through my headlamp switch either, or my dimmer switch, reducing their chances of failure, and improving voltage to the other signal light bulbs.
Don't discount the proper aim either. Suspensions sag, perhaps sending beams up too high, reducing apparent intensity on road, while glaring other drivers more.
I've been in a car that had a fender bender and one headlamp replaced. The output on the new non hazed light housing, vs the only slightly hazed original light, was extremely noticeable.
Part of that could be an old vs new bulb, but not all of it.