The downside that I see right off the bat is making a "smaller" turbo working harder to fill the plenum size. I could really see a smaller turbo running out of breathe way before the 400+ range with this intakethe only difference would be the amount of pressure (psi) needed to meet the tq/hp # of the previous intake setup. a small turbo will make 250 hp, the only difference will be the psi needed to make it, unless the flow path is so restricted that it needs such a high psi to reach the number that its out of its pressure map. it would be the same thing as people adding 3" intercooler tubes that are 4 feet long (at least) to reach a large front mount intercooler, only problem might be "lag" which so far usually isn't the case.Turbos arent sized according to intake tract volume, they're sized to mass air flow into the engine
there is also plenty of meat in the inj bore to be ported out to fit a heavily ported head like warrens that would reduce the "taper" point. again, this is why I opted for the unwelded version so I can port the intake flange to my head to have a nice flow path...