Early reports say the new IS handles pretty well around a track -
The stiffer platform improves both the ride quality and handling because it gives the new suspension a stronger base of operations. Up front, the double wishbone design has been retained, however, the stabilizer bar link has been reworked, which increases its rigidity by 20%. The IS’s new rear suspension was plucked directly from the GS. The new setup, especially with the optional active dampers aboard, does a very good job keeping the rubber on the road —P225/40 R18 front and substantial P255/35R18 rear tires on the test car. The steering has also been refined. Again, the starting point was the GS’s rack, however, further work has sharpened the on-centre feel and made the rack ratio faster. This brings better feedback without making the IS350 seem twitchy. It also adds much better straight-line stability. The final dynamic touch is found in the IS350’s larger front rotors and performance brake pads.
Testing proved the IS350 to be noticeably crisper. The steering is much sharper and it turns in with more authority. The improvements were made all the more noticeable on the rain-soaked skid pad. There was very little body roll and less understeer when the IS was pushed as far as the conditions permitted. A g-force meter backed up all of my seat-of-the-pant evaluations. On the wet tarmac, the current IS350 available for testing pulled 0.81g before the stability control system intervened. The new IS350 F Sport prototypes were pulling 0.91g before stability control intervention. Now, that is a significant and quantifiable improvement.
While the engines carry over from the current cars, there is an important upgrade for the IS350 — the addition of an eight-speed transmission (the IS250 retains the previous six-speed box). In the acceleration portion of the skid pad test, the current car put 103 kilometres an hour on the speedometer before hitting the braking zone — the new car with its extra ratios was showing 114 km/h when it hit the same point. The difference is found in the taller first and second gear ratios. They give a noticeably faster launch and a much stronger mid-range. The transmission will also have a positive effect on fuel economy. The canyon drive proved the new car is very quiet. However, an intake noise attenuator ensures the driver hears the right noises when it’s appropriate.
Engines are carryovers but as previously indicated they'll introduce a turbo variant during the mid-cycle refresh.
The 3.5 L 306HP N/A is now fitted to an 8 speed transmission instead of the 6 speed, although I don't know if that really makes a difference. The report above at least suggests it gives a "noticeably faster launch and much stronger mid-range" but who knows.
The 4GS GS350 is a better handling vehicle than the 535i (according to virtually every comparison) and the new IS is based on the 4GS, so who knows, it could actually be a decent product this time around as far as driving dynamics go.