With a ported intake/heads, there is less restriction and therefore less compression=heating of the air, reducing the tendency to detonate/preignite. The intake runner/valve is the bottleneck in the flow path between the intake plenum and the cylinder. Boost is the pressure in the plenum resulting from this bottleneck/restriction. Open the runner up, and the restriction is reduced, which lowers the "boost" number (backpressure). But the same amount of air, if not more, is entering the cylinders even with the lower number. The turbo is producing a greater volume of air in X time, than the runner can accommodate in the same time, so pressure builds up and gives you this boost number. Alot of people are fixated on "boost"...more boost is better mentality. They figure that porting the heads is no better than just cranking the boost up with stock heads. For one, you're being forced to push the turbo out of its efficiency range (more and more boost does NOT equal more air molecules in the cylinders because the air is heating up from excessive compression), and also that compression is creating so much heat that it requires alot more intercooler, alot more octane, and alot less timing to keep detonation at bay. If you study compressor maps, you can find out that the turbo is pushing almost the same amount of air at say 25psi and 35psi. 25psi is obviously better. You can open up the restriction in the air path to the point that you can flow the same amount of O2 molecules into the cylinders at say, 16psi, that would normally require 25psi. And at 16psi you can run stock octane with no alky and a decent amount of timing and have no knock....and your engine will be producing the same amount of power that it would have been before, pushing 25psi through stock heads. Boost is meaningless. Airflow is everything. That simple principle is what kenny duttweiler has always lived by and his records speak for themselves. His analogy is "you have 40psi in a bicycle tire and 40psi in a truck tire. Are they the same thing?
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