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Nezil, I've dug a bit more and my theory is this:
During shifts, if anything, the ECU is trying to SLOW DOWN the turbine when you think you need it more. Because:
1) Although DV dumps boost, the turbo shaft speed is still high. It's already high enough to support top end charge air (check MAF) which is too much for next gear.
2) The dumped (via DV) charge air goes into turbine housing, adding more pressure.
I'm not sure if BMW uses turbo shaft speed sensor though I know for sure Nissan does with the great VQ30.
ECU simply decides 80+% is the maximum WG duty to give when you need to SLOW DOWN the turbine, or there'd too much overshoot to deal with after boost target is met.
As RPM rises and turbine slows down, it gets to a point where turbine is not overloaded (for the RPM). Then 100% WG duty is allowed.
I do not think 80% makes a much slower spool up unlike what you see from your log. It could be the DV working in conjunction on 100% duty when allowed.
I've been logging boost behavior on DCT shift for a long time, because it plays a key role in acceleration and safety. My finding is that overshoot is a much more serious issue and takes 500-1000 RPM normally to settle. More boost headroom you leave, more poised the engine will be taking it up, hence the less power loss or you even make more power (by properly eating more boost!). Halim has a "boost guideline" tuning strategy (I made up that name and won't go deeper on it), with that there are even more uncertainties.
In any cases, the priority on shift is always boost dumping and turbine adjusting/decreasing. There is no way for 100% WG duty instantly on tap, unless you're already significantly under-boost before shift, which would be a much more horrible scenario.
In this sense, to "fix the lag" will be on less pedal down time. DCT fixes it by shifting impossibly fast, not by forcing 100% WG duty.
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Lemania 2320
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