Me too! Always room to learn.... but my wife and kids tell me that I'm so old I have a shelf memory.... when something new goes in, something else falls off! LOL
Joe, the NFGT shares a whole lot more than displacement with its mass produced cousins. According to Davie Pericak, 70% of the engine block and heads are in common with the 3.5 Ecoboost engines. In a different article, Raj stated (while referring to the engine in the Raptor), that 60% of the parts are in common with the NFGT engine. Both of these references are to the engine block and heads specifically and if we look at the numerous control systems (sensors, actuators, emissions control devices, etc) I'm sure we also can find a LOT of commonality.
But lets come back to the problem at hand....
For the life of me, I can't figure out why someone would pursue replacement of the wastegates in trying to diagnose what clearly? seems to be a release of boost pressure. Wastegates have nothing to do... at all... with bleeding boost pressure. Rather, the primary function of the wastegate(s) is to regulate exhaust gasses (not boost) passing into the turbo(s). By regulating these exhaust gasses, the wastegates can limit the amount of boost that the turbos can generate. Generally speaking, the engine systems are doing everything possible to maintain boost pressure. An uneven boost pressure would translate to a very "feelable" surging and it also makes engine management (AFR and timing) more challenging when it is not smooth and consistent. The OEMs typically do a very, very good job at this. The only time that boost pressure is dumped is in response to the driver's input of lifting off the go-pedal. So, you are at full throttle.... the turbos are at full song making maximum boost and suddenly the driver lifts the throttle and the throttle plate(s) are closed. That pressure now has no where to go..... hence the function of the blow-off valve.
OP - you have speculated that maybe the clutches are momentarily releasing and this is causing a release. I wouldn't think so. Shifting, particularly with DCT trannies is typically aided by engine management with a brief retard of timing. This is why you will often hear some nice popping in race cars as the quickly progress through the gears. However, boost is not dumped during a shift event. Consider that boost is kind of a precious commodity.... it's hard to get back. Although I don't know for a fact, I would doubt there's a plausible scenario of proactively releasing boost during shift events. (May be different at WOT when there's plenty of boost on-hand to almost instantly replace anything released.)
From you stated description and hearing the videos, my bet is on a faulty blow-off valve or a loose clamp or component in the charged side of the intake plumbing. I'm hoping we'll all know soon when you get this great car back in your hands!