Semper Fi. That is all.
It's just fundamental in the way a centi works. The centi works by spinning a vaned turbine that accelerates the incoming air outwards in a circular motion, thus why the inlet is in the center and the outlet points tangentially from the radius on the outside of the casing (wordy way of putting it). The efficiency of a centi is a product of the vane pitch and shape, some have vanes of different pitch/shape to widen the RPM range but lower the peak efficiency. So when you first get in it the blower isn't at its peak efficiency RPM so it's not making full boost, thus the lag. It's worse in a turbo because exhaust pressure is not linear with RPM, so you have to wait for the exhaust to spin up the turbo.
The roots and twin screw do not work by this same principle and don't have a low RPM lag.
Dan
If you are mechanically inclined and have access to just a few mechanics tools, save your self a ton of money and install the SC yourself. I did the Kenne Bell full kit (with a 9psi pulley, intercooler, etc.) with my wife helping and it went really well. We did it in my neighbors garage. You do not have to be a race car mechanic or NASA engineer to do the install. Just read (and follow) the instructions and take your time. When you're done, you have some bragging rights over those that installed with a credit card. Not that hiring it done is a bad thing, but it's cool if you can do it yourself...