The advantage to any split-type manifold or header on an inline six is the separation of exhaust pulses from adjacent cylinders in the firing order. 1-5-3-6-2-4 separates into pulses going front-rear-front-rear- - - - -etc. There is no "stacking" of exhaust impulses in the same pipe, so there is better cylinder scavenging with a dual setup. A measureable 10% power increase? Maybe in the land of unicorns and the Easter Bunny!
Cast iron split manifolds are fine- - - -RUN, do not walk away from tube steel headers! Don't go too big on exhaust pipe diameter on a split system because it will slow down the velocity of the gas in the pipes and reduce the scavenge effect.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!