I just ordered the engine from Gilbert Chevrolet and their race shop is installing the performer vortec manifold,harmonic balancer. 1 peice 153 tooth flex plate,water pump,fuel pump,etc at very good labor rate. I put the performer in to ensure my dual air snorkel fitting under the hood. Did I lose much in hp than the manifold tested with the engine? Just to mention also added the zz4 dipstick for proper fitment with headers.

 

The Performer is fine. From rumors I've heard, they used the Performer manifold for the peak torque rating, and something else for the peak HP rating. Who knows, though - just a rumor.