Fixed it!
Under WOT in 3rd at 4500rpm the Revs stopped building, there was a whoosing sound and turbo power was lost. There were no dash lights and the engine was fine when off boost.
I had "exactly" the same symptoms in my C5 when the lower pipe slipped off the right I/C so I was *fairly* certain it was another I/C pipe issue but I haven't seen this on any C6's yet.
Anyway, I opened the bonnet and the first thing that pops out at me -
The nearside intake manifold pipe had popped a jubilee clip - 30 second fix and something to keep an eye on. I may end up upgrading the pipe and clips on both sides anyway, just to be safe, as the other side needed re-tightening too and the pipe was slightly shorter.
I'm also thinking about doing the other two intercooler pipes as the lower ones were both very soft, probably with some black silicone pipes and proper aerospace clamps.