How much improvement you'll see with the harness installation depends on what sort of voltage drop you have in your existing wiring. Easy enough to test with a volt meter, probing the back of the headlight connector itself (with the lights on):

[Linked Image]

More info. including a chart of light output vs. voltage:
http://www.4crawler.com/4x4/CheapTricks/Headlights.shtml#VoltageTest