When it comes to rotors, there are 2 major considerations - we are trying to make an apples-to-apples comparison, so all rotors mentioned here are vented, OE or equivalent.
The 2 main things to look at are weight and diameter. You'll see an interesting twist in the assumptions we might make from this information.
The data below is from the manufacturer, in this case, Centric, which openly publishes it's rotor weight information. The actual data is taken from Rock Auto listings. There could be deviations in weight/mass from the same application(s) for OEM or other sourced rotors, but this is all I can provide for this discussion.
Other obvious differences are diameter, and rotor thickness--which means the 355mm rotor could be assumed to be the best at dissipating heat, since the air passages are probably slightly larger (32mm vs 30mm for the 321 & 345 rotors). I've not confirmed this--the rotor cheeks COULD be thicker, but the weight differences between 345 & 355 rotors may put this in question. In other words, the air passages could be the same between 345 & 355 rotors, but considering the revelation that the mass of the 345mm rotor is actually greater than the 355 rotor, I'm inclined to think the 345 rotor may actually have thicker cheeks, thus a narrower air path.
I've left the 370mm/6-piston combination out of this discussion, as it absolutely requires a larger wheel with greater lateral/outboard clearance than the 4-piston combinations.
When I stated that the 345mm rotor/Brembo caliper would offer "at least" 90% of the 355mm rotor/Brembo caliper performance, I had not actually looked up the data below. The one thing I am NOT sure of is the size of the pistons on the Brembo caliper used with the 345 rotor, but that would have a minor influence, if any, on actual braking force/power...if there is any difference in piston sizes compared to the 355mm Brembo caliper.
Centric G8 rotor - 321mm
Centric PPV rotor - 345mm
Centric GXP/Camaro SS/SS sedan rotor - 355mm
A little "math":
Rotor mass comparison
321mm vs 355mm - 24.5% increase
321mm vs 345mm - 26.7% increase
345mm vs 355mm - 1.8% decrease - favoring the 345mm rotor
MASS = thermal capacity - for a given amount of work (brake friction > heat energy > rotor temp change), greater mass slows rotor temp rate of change, reduces thermal range of pad/rotor interface, and/or allows more aggressive pad/friction coefficient vs smaller/lower mass rotor while operating at same max temp due to improved ability to transfer & shed heat
Rotor diameter comparison
321mm vs 355mm - 34mm difference = 10.6% increase
321mm vs 345mm - 24mm difference = 7.5% increase
345mm vs 355mm - 10mm difference = 2.9% increase
The other thing we don't know precisely includes what the clamping radius of the caliper-to-pad is, however, as already stated, both 345 & 355 brake packages use the same Brembo pad specs, so a figure of 5mm radially as the difference in the "lever" that will produce brake torque can be assumed. Certainly this matters in one respect, which is to calculate maximum torque potential, not always or often used in average street-tired braking events.
To be clear, I'm not a mechanical engineer, so it may be easy to blow up my statements. There are certainly other considerations to be made in a true comparison of the 345 & 355 brake combinations presented. Zero-out as many of the variables as possible - same wheels, same tires, same pads, same ambient conditions - and with the data presented, someone needs to convince me that the 345mm/Brembo package will not achieve 90% or more of the performance of the 355mm/Brembo package
, with justifications
As previously stated, I continue to believe there is clear evidence that both the 345 & 355 brake combinations will substantially improve braking performance over the G8 GT's 321mm brake package.
I think he's just trying to illustrate that the the DELTA difference from going from a 321mm dia rotor to a 345 mm one (24 mm increase 7.4%) is ALMOST as good as going up to a 355mm dia rotor (34 mm increase 10.5%) 3% less increase in brake dia. but the ability to still run stock 18 wheels. Is that 3% loss worth it to be able to use the stock oem wheels?
Question though. Dia is okay but is the spoke spacing still enough to clear the additional width? I assume these are a little wider
Second part of your comments--I have no actual data. The 345 caliper is a more compact design, so I am optimistic that it will work with G8 18" wheels, though I'm not aware of anyone that can confirm it.
So far, I know of one PPV with the 345/Brembo combination, in Elk River, Minnesota, running steel PPV wheels, which are already known to clear the 355/Brembo package--there may be others--so the objective would be to see if he or another PPV or G8 owner that has done the 345's can answer this, or would agree/offer to meet with a stock 18" wheel G8 owner for a test fit.