Longish post. Hear me out though!
TLDR; 355mm rotors found in a stoptech bbk offer negligible advantage over a proper rotor in the stock size–they have the same thermal mass.
Some ppl will know that I had recently sold my ST60 355mm brakes. The reason I did this is that I did not see a huge beit in going to these brakes. I was cooking fluid on stock brakes, and I was cooking fluid on the 355’s. It wasn’t a fluid problem (used SRF). I admit it was a pad problem (read: not using race pads).
Anyway, I decided to go back to the stock system and re-assess the situation. Pads for the stock calipers are less expensive than pads for the Stoptechs. Same with rotors and everything else. If I move up to an appropriate pad (planning on Pagid RS19s), I may alleviate the fluid boiling problem. Logically, one would argue that going down to a stock size is blasphemy. You lose thermal capacity. That’s the whole point of a BBK! Not to mention, stock brakes don’t vent well, and one side doesn’t even vent properly!!! Boo-hiss! But my logic is this - test out the stock brakes on a race pad. Only if it doesn’t work then move up to a BBK. Otherwise, I’m dumping 4k for no purpose.
So here’s what I did. I emailed Girodisc to inquire about their 2 piece rotors. Yes, they are spendy, but they vent properly, the air gap between the 2 sides of the rotor is larger, there are more rotor veins internally and the veins are curved AND the metallurgy in the Girodisc rotor is the same as those found on motorsport rotors. This is not your stock rotor. But wait. More boo-hissing. Why is boro spending so much on crappy stock size rotors???
What about thermal capacity? Afterall, stock is 345x30 and my old BBK was 355x32!!!
Mike at Girodisc did me a solid. He dug up the rotor rings from his inventory and compared them. Stoptech 355x32 ring replacements weigh 16.4lbs and the B8 345x30 ring replacements weigh 16.2lbs!!! Read that again. The difference in mass of a stoptech 355mm BBK and a proper 345mm rotor ring is only 0.2lbs – 90g!!! But wait!! How is that possible? The stock rotor is so much smaller! It’s because the pad on the stock caliper is deeper(thicker… more annulus) . So the stock rotor ring is thicker than the ring on the 355 stoptech–this is where it gains it’s thermal mass back.
So here we have a properly venting rotor–the same design as that found in a BBK. It’s 345mm yes, it’s thinner too. but it trades virtually nothing in thermal mass. So arguably, moving to a 355mm BBK actually does you absolutely nothing other than improved pedal feel and less caliper flex. The 355mm kit for 2.5k or whatever just buys you pad selection, stiffer caliper (better pedal feel)…and that’s it. The running costs of maintaining a BBK and replacing consumables is far greater than the stock system. For those of us who track our 4k lbs pigs, you understand that consumables on this car suck and are eaten quickly. Reduce cost where you can, spend it where it matters (more track time 
:D)
FWIW the Girodisc rotors are actually the same casting as the Stoptech rotors. So design wise for air gap, # of veins etc, it is the same.
So the real word on BBK is…if you’re going to do it on this car, do 370/380mm size. 355 offers advantages, but knowing this info, it’s more bling than anything else.