Rocket Scientist,
The easiest way to control boost is with the pulleys, if you want to do the daily drive thing, put on your 5/6 lb pulley, when it is time to race throw on the 8/9 lb pulley, it only takes ten minutes to do a swap.
No one had mentioned Powerdyne, this is a great mid range blower, I have been running it for six years and it makes nice horsepower, powerdyne has 6,9,11,13lb pulleys, and it doesn't require an oil tap and they are pretty cheap.
They have double shielded ceramic ballbearings that are rated around 45-50 thousand rpms, although these are the guys that will go and if they do they are about $145 each and you will need two.
The throttle body plate allows the air to come in, so you don't get much boost until it is open, most recomendations are that you don't need a by-pass valve until you make over 9lbs of boost.
Centrifugal superchargers are very linear in their rate of boost, based on rpms, but roots style superchargers make upwards of 85-90% of their boost at low rpms and don't make much more after that, it could be still considered linear but not as noticeable as a centrifugal blower.