The reason the ls tranny is concidered better for the turbo is the spacing between the gears. The longer spacing gives more time for the turbo to run. Typically auto trans cars with stock turbos are faster than standard with stock turbos. This is because the turbo spools up and stays there as the tranny shifts gears. Every time you back of to shift gears your turbo has to spool up again to start making power again. The short ratios in the gsr tranny means you shift it more which means less time with the turbo making power.