The IS350's are fast. I have the IS250 version, but both are V6 powerplants. One is bigger than the other. On the IS350 they use a combination of port and direct fuel injection system not seen on most cars to give them that extra power. On mine, I believe it's DFI only but some people say it's a hybrid of port and direct injection too like the IS350 (which I highly doubt). The only difference is that I'm getting better gas mileage for the performance and a significant HP reduction since it's a 2.5L block. Too small for my taste, but Lexus need a small car to fit the market. I drive a lot so gas was one of my main concerns. I could've opted for the IS350, but the fuel consumption is different.
The chassis is literally exactly the same and it also shares it with the new 2008 IS-F 400+HP V8 since we're all in the same "IS" family.
But the IS250 acceleration is very potent. We have the option of putting the car in Power Mode, then using the shifters (since it has a sequential transmission), it can rise to the occasion. When you redline it, you can hear the supercharger like sound "shhhhhh"--almost like a whistling sound. I love it. You know that Nissan commercial where they showed the Altima accelerate and you can hear that shhhhh sound? It sounds like that. I have no clue where it's coming from but it must be the pressure buildup or something that causes the sound to release.
An exhaust system for this car along with many other axle back systems will have virtually "0" gains in HP other than sounding good. I could use it though to drown out the injectors tickng all the time