[urq] turbo question

[SpotatAshleys]

Evening chaps
A friend of mine who has no access to internet has asked me for help. He has a sport quattro replica (lucky chap) but has recently suffered a series of kkk turbo failures (not so lucky). In all instances all damage has been down stream of the engine i.e. no damage to the engine. What happens is the exhaust side explodes (literally) throwing bits of metal everywhere. 
The engine is a finely tuned 20v, running modest boost but dyno'd at about 500bhp - prior to all this. The failures have come after virtually no miles. In one case, about 20 yards off the start line after last rebuild.
The oil supply to the turbo has been checked and given the all clear. No bits in the oil. Datalogging shows no curious changes in mixture (leanness). The things just explode. All bits go down stream. Engine strips revealed no damage - I think I said that already.
To me it seems like exhaust isn't flowing enough when at WOT causing pressure build up, heat build up and bang. Doesn't seem possible though. Prior to each failure, it runs smooth, no smoke, no noises, no water temp or oil temp rises. Weird.
Any ideas what could be causing this? His bank manager is starting to ask questions....
All ideas gratefully received.

Cheers

Keith

'87 WR