When I first did the rst conversion on my cabbie mine used to chatter like mad with MFi with the boost only set at 10psi. It seems to vary from car to car. With MFi the flap on the metering head closes as soon as you take your foot off the throttle so chatter can't be heard. Chatter is turbo stall which can't be good for a turbo long term (in my opinion anyway).