Sky34, I agree with Gav, what you've been told is not true. I know at least 4 De Jetro tuned GTR's in WA which have no problems with idle or fuel consumption. There is a trick to getting the idle right with multiple throttle bodies. I'm sure the tuner you spoke to just doesn't like De Jetro's which is fair enough, but they are a very good product for high hp cars.
Cheers
Paul