The upshifts are reportedly 0.3 seconds - the same as an IS-F.
You ask how the third-generation IS 350 engine compares with the second-gen IS 250 ... I assume that isn't a misprint. One would assume that a larger capacity V6 is going to offer better acceleration over a much smaller-capacity V6 at any given engine speed ... but let's look at some figures to prove it.
Regardless of the transmission, the new 350 has the same engine as the previous IS 350. So the relationship between revs and power will be the same, although acceleration will be 2-3% slower due to added weight. That means we can rely on any power curves published for the IS 350's 2GR-FSE engine, no matter how old they are. In those power curves, it can be seen that at, say, 2500 rpm, the 350's engine produces 110-120 kW of power, making it more powerful than the 335i's powerplant (although much the same acceleration due to a 100 kg weight penalty) and slightly more powerful than the supposedly "torquey" Audi 3.0 L TFSI engine (and accelerates faster with the 350's lighter weight compared to the A4). This is in comparison to the IS250's engine, which produces ... um ... around 45 kW at 2500 rpm. But the faster shifts and G-AI mode in the new IS 250 definitely improve the experience over the previous generation IS 250.
hm.. 0.1 on the isf on upshifts 0.3 on downs