I've already decided not to use the 4.10's, but I may still go with the 3.90's. I have to get it to the track first to get some trap speeds and play with shift points before making any gear decisions.
As for the turbine housing, I understand what you are trying to say, but RPM is most certainly a factor. There are many ways to look at this. (And it's Area/Radius, not Air/Ratio)
1) Big turbine housing, higher A/R. More potential flow for peak HP, but the tradeoff is the car must be revved higher. To get into that powerband faster one option is a lower gear.
2) Smaller turbine housing, lower A/R. Lower potential flow for peak HP, increased backpressure, lower powerband, but could also keep current gears if desired.
3) Smaller compressor wheel. Lower potential HP, moves the powerband down. Boost builds faster.
Of course this is oversimplified. An overly large compressor side cannot be overcome by an overly smaller turbine side. The only option there is more RPM, or a smaller compressor side. I certainly don't want to spin my 4.6L like a Supra if I don't need to.
Garrett has a wealth of information on their site regarding
compressor and turbine A/R.
The problem I face with this car is that the kid who put it together had no specific goal in mind other than to put a big turbo on it. Now it's up to me to balance the system to how I want it, and I won't know myself exactly how I want it until after I get more seat and track time.