i don't know where you've been looking, but actually most boosted IS's have supra 5-speed or 6-speed manual trannies. and many of us actually went from auto to supra manual. not always stock manual to supra manual.
That's because on our cars, the auto transmission can withstand more horsepower than the manual can. Also, 5 speed manual IS300 are rare.
IF your planning to have a 500+ hp IS300 then switch to auto. IF not keep the manual.
somewhat true, about the auto trannies being able to withstand more power than stock manuals. its true if you upgrade your auto tranny, with sprag clutch, valve body, tranny cooler. if you keep the stock auto tranny as it comes, its not stronger than the stock manual tranny. and it might actually be weaker (there have been some members running 400hp on stock manual trannies. i wouldn't recommend it! but its been done)
but to withstand true power, your weakest link will always be your stock tranny until you upgrade to a 5 or 6-speed supra manual.