I think usually the added thickness is due to more protective coats/thicker coats of silicone rather than actually thicker gauge wire. i dont think it effects voltage but rather keeps the heat from destroying the wire. but overall it would make the plug wires last alot longer...
I agree with the durability comments, but apparently a thicker wire does have the "capacity" to deliver more voltage (according to Magnecor). But we know that resistance will limit that "capacity."
This is an interesting issue...so I did some research...first, on Magnecor wires.
The 10mm wire has more silicone than the 8.5mm. It has a 2.5mm conductor. Each wire can deliver 80K volts with 2,200 Ohms/ft. resistance. The 8.5mm wire has a 2.5mm conductor and can deliver 60K volts with 2,200 Ohms/ft. resistance.
Our stock ignition system delivers somewhere between 120K and 180K volts. I do not have the factory specs, but based on what some of our esteemed mechanics have said here, that is probably the range.
So, even the 8.5mm Magnecor wire has more than adequate "capacity" to deliver whatever voltage our ignition system can generate. However, I know that at 2,200 Ohms resistance, Magnecors will not deliver as much of the spark as a wire with less resistance.
Then the question is how much energy is needed at the plug to optimize fuel burn? Another is, what is the maximum resistance that will still allow the wires to carry that amount of energy? I have no idea...maybe 2,200 Ohms is just fine! So I made a decision in my blissful ignorance of hard electrical data.
When I decided on the Bosch 8.5mm at 500 Ohms/ft. resistance, I just assumed that less resistance is desirable...and that seems to be popular thought. Plus, I needed new wires because of corrosion at the terminals.
You should know there are some other low resistance numbers out there: Pertronix claims just over 500 Ohms; Accel 300+ Racing wires claim 150 Ohms; MSD Super Conductor is 40-50 Ohms. Of course, other issues include whether those wires are made for our cars and at what price.
Regarding thickness, there is no question - assuming silicone jackets of comparable quality - that a thicker wire will dissipate somewhat more heat and probably last longer than a thinner wire.
HTH
Steve