Tim,

Two things come to mind, First is that the turbine and/or the housing A/R is much to small. Second - the Backpressure will most likley climb very fast and create valve float and other issues before you get in any usable RPM band.
If possible, I would try and plumb a gage port in at least one of the manifolds and monitor backpressure. In a perfect system, It should be near a 1:! ratio around your usable power band. It looks like in your case the Backpressure is going to be way to high and climb while your boost drops.
Another thing that may happen is turbo overspeed. You may bust the compressor wheel if you spin it up to fast with that backpressure.
Allan G.