It has to do with reaction time. When you accelerate hard, pressure increases rapidly. There could be a lag between the time 15 psi builds in the manifold and 15 psi registers at the wastegate actuator - which is another meter away if the signal line taps into the far end of the maniold - at which time the manifold could be at 17 psi. So you set the wastegate to open at 13 psi. Well, when you accelerate less aggressively, the actuator will see 13 psi about the same time as the manifold so you'll never get to 15 psi you want under those conditions.

This thinking comes to me by way of engines that rev more quickly and to higher rpms than MB 60x engines. Said engines were very sensitive to manual boost controller location and setting. I had one engine that peaked at 16 psi in 'normal' driving that then spiked to 22 psi with the pedal to the floor in lower gears until I sorted signal line routing, boost controller spring tension, etc.

Try it the 'easy' way first. If you don't see boost spikes driving the way you normally drive, pat yourself on the back and go on to the next project.

Sixto
MB-less