Which conversion are you talking about? If its the old Janspeed turbo with the 2" SU on the front, then I ran one for about a year so might be able to help.
Well after an extensive road test of my friends 240z turbo I can say that it's in my view no faster than my triple webber set up. This version was the Janspeed turbo running a single 2inch SU on the front the power delivery was smooth with plenty of punch through and over 100mph and it has been to 140mph. much quieter than a triple set up but I love the noise my makes. As I am also thinking of building a turbo motor I think I would go for a injection system type turbo but then the money starts to mount up big time.for more info on building a turbo motor see www.hybridz.org
The car I test drove is up for sale if anyones interested I can forward on his number.