[XutvJet]

There is no easy answer here as these are complex turbo motors. These motors have an electronic waste gate, don't use a throttle body, and have variable cam timing. BMW can greatly manipulate the shape of the power curve and the way power is delivered. BMW strives to make these motors feel naturally aspirated and significantly modify the boost and timing curves, especially in the lower and midrange rpms. It's also why most tunes pick up significant power in this range.

With respect to load, again, it's a very complex matter because these are turbo motors and not naturally aspirated. The engine simply doesn't have to pump as hard in the lower gears because of torque multiplication at the gearbox thus less boost is generated because the engine doesn't fill the cylinders as much thus can't create the exhaust velocity to spin up the compressor rapidly enough to get to full boost. Think of a multigear bicycle. The bike is the gearbox and you are the engine. Go up a moderate grade at 10mph in the short gears and your legs and lungs don't work as hard and it's relatively easy. Put the bike in a tall gear and go up the same grade and speed and you'll struggle. You'll breath in a lot harder, breath out a lot harder, your legs get stressed (think connecting rods), etc. This heavier exhaling is what would spool up the turbo.

The slight hiccup in upper rpm power most see with tuned B58's (especially piggybacks) is likely the DME trying to make sense out of what it perceives as wonky data from sensors when compared to what it is expecting to see, thus it manipulates timing, fuel ratio, bleed boost with the EWG, etc.

If you want to really understand the full relationship between load and boost, there is plenty of information out there by simply searching "turbo boost, load, gear". As you'll see, it's very complex and highly variable.