After 33,000 mles driving the IS-F and 13,500 in the RC-F, I have found the RC-F to feel faster in almost all road conditions, except the wet. It also has a greater sense of occasion and is no problem in car parks. The back seats are a little tight though.
The RC-F does not feel heavier to me, possibly due to the improved power to weight ratio. I also prefer the steering, which is heavier. The IS-F did feel a little less attached to the road in a corner, which I imagine translates in the eyes of journalists into a looser back end and more drift potential.
The biggest improvement is the ride; it is a step change better.
I would love to get back in an IS-F to see if my memory is playing tricks on me. Now I've really fallen for the RC-F, there is a risk all sorts of biases have taken over. I've even decided not to swap it for an Alfa Quadrofoglio for at least another 2 years; it si that good.