You are correct that the stock turbo will not maintain boost at high RPMs, but it is not because the turbo is flowing less. It is because the engine is flowing more. In the case of the engine flowing more, you would expect to see hp gains, but you don't because of the items I listed above.

 

i noticed a lot of talk on failed trannys.
About how much power/torque do the stock lnf trannys hold?

 

You can break it with just tuned power levels, depends how often, how hard, and how good/bad you drive. Plus the tires suspensions mods etc that you are using.