Here's the best way to think about it, IMO
The power you make is determined by the average pressure over the course of the powerstroke from TDC to BDC when the pressure times surface area of the piston is placing downward force on the connecting rod, turning the crankshaft, imparting torque into that shaft, the rate of which is your power output.
Well..... on the next stroke from there (BDC) back up to TDC again the exhaust drive pressure is atop the piston. So you have that average pressure times the piston surface area taking AWAY force/torque/power. The lower the drive, the less power your rob, or the more power you make, all else constant.
Furthermore, there's the issue of negative effects to cylinder VE. When the boost:drive ratio is bad out of whack, during any valve overlap period, the exhaust gasses will actually revert back into the cylinder from the manifold, right across the top of the piston and back up into your intake runner if things are bad enough. Some people have actually noted SOOT up in the intake runners. #1, this means that for any given displacement you HAD from the stroke x bore of the cylinder, you just pissed that much away because it never gets filled with fresh air, it's still getting sloppy seconds from the last combustion event still just hanging out taking up space that fresh air could have used. #2, it raises the in-cylinder temperature (burning gases present) potentially raising the temp enough to have a notable effect on the effective start of combustion, or in essence, advancing your timing.
This is where people like to point the finger at drive pressure for blown head gaskets. It's not the pressure, but the potential effect to cylinder temp, and it's subsequent effect to ignition delay..... or lack thereof as the case may be.