I think you need to forget header theory when dealing with a turbo as the turbo will be the biggest restriction in the exhaust, think of log headers versus tuned headers. tuned headers don't gain much over a log design when pushing through a turbo.
So cylinder #1 and #7 join up with #2,4,6,8 under the engine and then they all flow up to #3 and 5 and then out through the turbo.
