Zero vacuum is not needed to have good shifting. In fact, the only time that you approach zero is at maximum boost. My personal opinion is that 2 inches of vacuum will do, effectively, the same thing as zero vacuum, if you can ever get there.

You can, of course, prove this to yourself. Just disconnect the line to the transmission and you will have zero vacuum. The shifts should all be very hard, to the point of banging the drive train. If they are not, then, you have other problems.

When the SDL had vacuum leaks, this is exactly what it did. The vacuum to the transmission was much lower than spec.