Tyler,
I have no doubt that Precision did their homework with the new 66 Turbine Wheel.
A shop out of Chicago made 852RWHP with a 6266 on a high compression E85 Supra Motor.
These motors spin a lot of rpm and are a lot more efficient than our combinations.
However most 3.8 Buicks don't push their combinations enough to even max out the 62 Turbine wheel.
Those people would not go any faster going from the 62 to 65, let alone the 66 Turbine IMO.
Someone willing to run 30 plus psi and has a motor that makes power past 6000 rpm would see a difference.