The internally wastegated housings only waste the rear 3 cylinders, so you're going to have more restriction on half the engine. The internal port is only about 1/2 inch in diameter, so you can actually overcome the wastegate if you are burning enough fuel. The problem with a single turbo with a tiny turbine housing and a wastegate is that it becomes very inefficient when you need to flow lots of exhaust and air. Spinning a small turbo very fast is an ineffiecient way to flow large volumes of air. You wind up pressurizing the air more by heating it than by compressing it, which means your boost number looks impressive but you don't have enough air molecules for your fuel molecules. So at a certain point twin setups become necessary for driveabilty and power.

The 21 cm is really too big. It was a mismatched turbo setup to begin with, I don't understand what Cummins was trying to accomplish when they set up the early intercooled truck that way. A turbo works through pressure drop, and the 21 cm housing is so big that it just doesn't build enough back pressure no matter how much fuel you throw at it. The compressor side actually would become a restriction to the volume of air required to keep the EGT's under control if you had the fuel to spool the pig. It wouldn't make enough boost because the compressor couldn't suck in enough air.

The whole point of twins is to be able to supply the engine with a large mass of air, not just high pressure, and at the same time keep back pressure to a minimum. You seem to have the theory of operation straight on the exhaust side, the primary wastegate wastes around the primary, not to the downpipe. On the intake side the air comes into the secondary (big) turbo first, then into the primary (little). The primary turbo is being fed 25 or 35 psi of boost which it then takes to 60 or 80 psi.