Turbine size is not a wrong or right thing really, I mean you can have the wrong size for a specific application but that's about it. A good way to consider it is how will the car be used 80% of the time. Then try and size the power band around that. More often than not unless you plane to move a high volume of air (high boost, large ports, high rpm) a .84 on a 66 wheel is very good for a responsive 450whp street car with boost around 15psi-20psi and a 1.0 is very good for the same on the track, while a 1.0 is about where you want to be if you're going to run around 18-25psi on the street and anything over 25psi on a well ported engine you may want to start looking at 1.15's.
Those numbers can vary depending on a few different variables but once you reach a given boost level you can start to be a restriction, at 25psi you tend to be a restriction with a 1.0, are you making power? Sure, could it be making more? Likely, will it be more laggy, minimal if boost is at that level as it will ramp up much quicker.
All of this will change once you change turbine wheel size as well. A 1.10 on a 96mm turbine will breath a lot better than a 1.32 on a 66mm turbine.
~S~
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