I recently bought a 2006 IS350 and find the handling to be very good at speeds below 60. Above 60 it becomes a little twitchy.

Wouldn't stiff sway bars increase this over-responsiveness at high speeds?

Also, at low speeds, I am very content with how little body roll the car has. The suspension must be in very good shape as feels better through onramps/offramps and around corners than my previous A4 which had front/rear sway bar motorsport end links, huge rear sway bar, and fairly low on coilovers.

Wow!

 

that is exactly where you see the benefits of having a thicker rear sway bar. It provides greater turning stability at all speeds. A rear sway bar is single handily the best mod if you ask me. A strong chassis is critical here too but not much you can do in this regard.

You will see pretty greater gains since the is350 is equipped with a thinner rsb than the is250. I think the is250 and 350 share the same upgraded sway bar.

 

I'd recommend the sways for sure for better handing. Folks can take offramps with more confidence at higher speeds with the rear sway bar. Getting both front and rear sways is the best route if you can though.

 

I did f sport front and rear sway bars with f sport springs. I would absolutely suggest it, as the upgrade is amazing. It was good with just the sway bars, but even better with springs.

 

Sways bars can make you over confident....don't forget sticky tires or kissing the wall on off ramp