Was the car cold? I think there's a performance limiter before the vehicle has reached proper operating temperature.
The IS300 isn't very powerful off-the-line. Where its drivetrain seems to shine is at-speed (40-100mph).
The IS300 automatic transmission does not perform well when left in "D". It gives a very smooth power delivery for everyday driving, but damps the effects of sudden accelerator movements (drive-by-wire and ECT-i) and seems genuinely confused when the pedal is put to the metal. With the transmission in "M", pretty good performance can be achieved if you E-shift correctly. This is not as easy as it seems, since the transmission will downshift out of your gear selection (without indication) for you if the depression of your accelerator exceeds a certain point. It also doesn't help that you cannot select first gear.
From a stop, you would have to select "M2". As you mash the pedal, you look at the tach and wonder whether the transmission has decided to go with 1st or 2nd. If it's in 1st, you obviously wait until it shifts to 2nd and goes through that before you would have to do anything. But what if the transmission is already in 2nd? You would bump up against the rev limiter if you failed to shift, thinking that you were in 1st. The way to work this transmission properly (my opinion at this point in ownership) is brinksmanship. If the tach seems to be going all the way up to the limiter, you're probably in 2nd, so you would need to upshift RIGHT NOW (attention and fast reaction times help). The assumption is, being that 1st is a very low ratio, that even floored, 1st to 2nd does not occur at the maximum rpm.
At speed, you would downshift until you see that you have exceeded the gear that the tranny was actually in. The tach would jump, and the time difference between that and punching the accelerator would be engine braking. Again, good reaction times help minimize this.