yeah my brothers Fiesta RST has had the dump valve plugged, however i am pretty sure he has an external wastegate, thats why he gets the lovely rally style flutter sound, sounds great.
however on the 1.8T its just a silly idea, i remember my mates old man's golf 1.8T started making flutter noises and sounded really cool. the standard dump valve had broken down and blcoked itself ( not that he knew that ) lasted about a month before the turbo gave up the ghost.
i also know when i had my adjustable spring on my 1.8T and did it too hard, i also got turbo stall, sounded good but i am sure its not healthy.....lol