ive not seen one hold that much, we do lots of flat out t34 maps, and the gate is closed after the spike has been controlled all the way to the limiter, and the engine takes all the air the t34 can make, the final figure is dependant on how much of the air the engine is using, because the turbo is flat out,

its quite interesting watching the boost gauge on a flat out map, as you can see the spike,as initially the turbo produces more air then the engine can swallow, then watch the gauge slowly drop as the engine consumes more and more air as revs increase, you could weld the gate closed and not see any more plenum pressure lol