Wing, I would rate the TSX steering firmnest up there on par with a Mini Cooper S with 17" tires, and the RX-8. How does the IS steering compare relative to your S2000 and your previous RX-8?
For those wondering why many are concerned with steering firmness/feel, it is like a channel of communication between the car and the driver on front end grip and workload. Light steering is like whispering, communication is dampened. Although firm steering doesn't necessarily equate to better feel. The ideal is a linear correlation between steering effort and workload on the front tires, so that the driver can sense this along with the seat of his pants. This feel can depend on suspension geometry, tire contruction, amount of assist on power steering, etc.
The IS250AWD i drove had 17" vs. 18" in your tester. You must've driven the IS250AWD and IS350 also; do you notice a difference? At one point I was considering an IS250 just like your tester, but was turned off by numb steering feel of the IS250AWD demo that I took out (they didn't have a 250MT at the dealership). If indeed there is a difference between your tester and other IS variants, I think it'd be nice to know as many who are interested in the IS layout (sporty) would be turned off by its numb steering (typical toyota).
I will say that steering response was quicker in the IS I tested than the TSX. It felt to be a result of a lower steering ratio, rather than a lighter front end, like that on the RX-8, which turns in extremely well.