My set up and advice please...

[BMWTurbo]

So further to this the engine does the same without the W/I turned on. I hadn't noticed it with the W/I turned off the other day, but further testing shows it does. So it's not W/I related.

I've regapped down to 0.6mm (24 thou) and it's basically gone now even with W/I on (made no clear difference). I'll probably look at moving the nozzle back up to the TB because it's a 'nicer' place for it to be physically and doesn't appear to have been the problem.

I think it's purely down to cylinders pressures. When the second turbo comes on the boost ramps up very hard and peaks at 25.5psi, whereas later in the RPM range is settle back down to 23/24psi which is where my boost solenoid control points are.

This explains why I could replicate it by backing off and hitting it again in 2nd gear as the boost ramps up harder (and peaks higher) with WOT in 2nd from cruise than it does in 1st gear WOT.