I hope that when you say that you are going to run the "relay" section of your circuit through the switch, that you are talking about the coil side of the relay and not the contact side. The coil side will have minimal current draw.
Fabricating a high current wiring system with relays and heavy wire will indeed protect the wiring of the vehicle and should be considered absolutely mandatory if you're going to use 100W bulbs. However, by decreasing any wiring resistance, the voltage at the bulb will increase, which will decrease bulb life. The wiring beef up will not extend bulb life, it will probably slightly decrease it, while it is increasing light output. If this were an inductive load such as a motor, lower voltage would decrease it's life, with a bulb, however, it's life is strictly determined by heat dissipation. If there is more voltage AT THE BULB, there will be more heat to dissipate. This will also, obviously, create more heat within the lamp assembly.
I bought a parts car with European lights which I then put in my driver. They had 110Watt bulbs in them, and smelled of high heaven with a burnt electrical smell. I put 55/60's in them and they work great. The shape of the beam has as much to do with light output as the wattage of the bulb. That's why Euro lights are so much better.
If you had some data on the different types of light bulbs, which would tell you the percentage of energy which generates light, and the percentage of energy generating heat, you could probably derive which bulb type could be tolerated with the least amount of heat dissipation.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in