I've just had a watch of Jon's video of your car on the RR and I'm pretty sure I can hear the engine backing off just before the drops appear on the live curve being plotted on the screen.
I stand by what I said on Saturday - it's probably overboosting, which is being sensed by the ECU and the power is being backed off. Whether this is in the ECU or on the physical wastegate valve inside the turbo is something you couldn't say without knowing what was done in the mod.
I'll try and explain;
When I had my remapped Volvo, I adjusted the wastegate out of curiosity and the results for such a small adjustment were staggering. This worked both ways - adjustment one way meant there was no power at all and adjustment another way meant there was a massive amount of power but in the wrong place.
Here's a very simple diagrammatical explanation of a boost valve & wastegate; http://johnmonnin.netfirms.com/image...BallValve3.jpg
Basically - the tighter the bolt on top is, the more turbo pressure is allowed to pass through the turbo spinning the turbine and causing boost. Too much pressure can be harmful to the turbo or the engine if the fuelling isn't up to it.
I don't know what Mangoletsi's turbo mod involves, but it could have been an adjustment of the wastegate to allow more turbo pressure and it's possible that because the ECU hasn't been remapped to suit, that there is excess pressure running through the turbo (and being sensed) and the fuelling is being backed off. It's also possible that there is an ECU controlled wastegate which will open if the boost levels are too high which would result in the same thing - a drop in power when overboost is detected.
Hope this helps to explain it a bit, although I'm guessing at why the engine backed off - it could be totally unrelated, but the symptoms fit with an overboost.