in regard to the downstream question, i am staying with stock for the time being. i like the idea of the unit that W+N sells - except i have a 3.5, and probably always will ... perhaps it will be changed to a b35 instead of a b34 ... or my head will get worked on to make my b34 more robust. there's always the coupeking piece. i will wait until i get a cam / headwork before i worry more about the exhaust. i will be going with either a schrick 284 or a paul burke 272 or 284 (not sure of the actual numbers) ... not the radical 300+ cams.
i know that Jere Stahl was king of the long tube header ... not sure about the small diameter thing. interesting, the articles also compared headers by ed as a long tube guru along with stahl. the article by ed was enlightening.
don't care too much what the thought on the e28 board is ... it depends on where you want to make power, and what you want the throttle response to be. for me, i want an increase in power below 4000rpm. so that's is smaller tube. if your goal is ultimate power at the top end - go with the 1.625. the biggest i would consider is 1.5, but i think i will be happiest with 1.375. that's what the articles that were posted led me to believe. that's what korman's people tell me. bigger is NOT necessarily better in this case. but its your money, your dreams, do what you want and what suits your goals.
cheers