Your right on one thing, all turbo cars produce slightly less boost on the dyno than on the road, but where the big twins suffer is with building boost.
My car has been to SSR many times and I have spoken to Charlie about the difference in the dyno graph to how the car drives on the road. He informed me that the dyno does not load the car as much as driving it on the road and my car will suffer more from this than a single car, even of the equivalent size turbo.
Having 3 cylinders running each turbo requires more exhaust gases to get the turbos going, so this raises the boost threshold a fair bit on the dyno.