685hp seemed high to me too but considering your numbers it might be possible without the SAE vs STD dyno tricks.
I think his 23% over your 18% would nearly equal out your headers.
He has a manual trans which would give him what, 15hp over our automatics?
That would put him right at 685 then.
Op pully it up and put on a
AEM V2 Water / Methanol Injection Kit
https://youtu.be/y-1giF7jQfg?t=453 e85 vs methanol injection (or both)
I want to see what it would do so we might follow your lead.