Red9, your missing the point. The most important thing is to choose a turbo or turbos which are suited for the application. What I mean by this, lets say your chasing 600hp, you could choose one Gt35/40 which is rated at 650hp or run two T28's which are rated at approx 300-350hp. Both of these would work fine. One thing to remember is when you run twin, you only have half the energy to run each turbo and they need to be sized appropriately. If you get the sizing wrong on a twin application and go too big, the lag would be way worse than a big single. In an ideal world, I would run twin on a V engine and single on an inline engine, and I would keep the turbos as close as possible to the exhoust manifold. They all have their advantages.
Attached is the dyno sheet of my GT40. The boost control is taken care of by the Haltech ecu. I also tried a friends Turbosmart E-Boost 2 boost controller and the boost came on much earlier (about 400rpm), and I'll be changing to that soon. We let the car coast on the dyno at about 2,500rpm with no positive boost pressure and then we floor it. With the E-Boost controller, we had 0.5bar at 3,300rpm and full boost at just over 4,000rpm and that is with a motor with stock standard heads, valves, valve springs, totaly untouched heads. We achieved that using Shell V-Power with 5% ethenol. One thing we found is the stock valve springs are at their limit and start to float at boost levels above 20psi.