as far as im aware the reason for them sticking is that they were taken straight off a diesel engine and not designed directly for a petrol engine, therefore not prepared for the petrol heat,
also the soot produce in a diesel engine wont help.

jay i would give more details if i had them but i mafraid i dont, and no i havent sold the 21 or any other car. lol

as RWD says the vnt only really does its thing lower down the rev range, to help with spool up.
once on boost surely it wouldnt change the backpressure??????

if it can be asfely controlled during lower boost levels (while spooling up) with lamba, backpressure sensor, exhaust temp sensors....
then it has to be worth a try, how else will any1 find out?