noooo, not plenum pressure, COMBUSTION CHAMBER PRESSURE!
the pressure in the combustion chamber (tdc, valves closed) as the intake charge is burning, after your 9.6:1 compression (remember pv=nrt?, so much more heat=much more pressure)
now, if its 1.5x as much intake charge as before, its gonna burn hotter (and make moar powar!!11) and have a higher pressure. nitrous does the same thing, cept its increased oxygen content is not thru >atmospheric pressure, but through a chemical reaction (breakdown of 2 N2O into 2 N2 + O2)
what im trying to say is,
your pistons have about a ~100chp window above stock, 7psi boost isnt gonna punch through that, but 7psi+75shot sure as hell will.