There was a long discussion about this on "another board", and some folks got a little excited. One post said that going to a 70mm TB and the matching plenum added 3psi, with no other changes. Here's something to think about, though. Carburetors are rated for flow at a very low pressure drop. A Holley four barrel is rated at less that a one psi pressure drop, for example. Now think about the size of a 1000 cfm Holley, with throttle plates and shafts, booster venturis, etc, blocking the flow. If the Holley will flow 1000 cfm with less than a one psi pressure drop, what does it take to flow much less than that into a little Buick engine? Remember, the bigger turbos may flow 1000 cfm on the INLET, but it gets compressed to 500 cfm or less at the outlet. I agree with Steve.. the TB won't be a restriction until you start to flow a LOT of air.