Have you checked that your boost isn't falling away at higher revs (as it did with the stock DV in the MM video)?
The other thing I'd be checking is fuel path (no clogging/reduction in ability to flow fuel into the engine) - if that's happening, the ECU will detect the car leaning out (or will detect the start of pre-detonation) and will slow things down to ensure there's no damage to the engine).
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