While I don't have an IS 350, I have an Acura TL with summer tires - 235's. In the snow, they are horrendous. I mean terrible and unsafe. I just installed some dedicated snow tires - Blizzak WS-60's. The difference is night and day. There is a huge difference between summer tires and winter tires in terms of snow traction. In my mind, this isn't really about the fact that the IS 350 is RWD. The IS 350 has high-performance summer tires. These tires are wide and have a tread compound and tread design that was developed for uncompromised summer performance. Everything that makes them perform in the summer makes them terrible in the snow - they are wide, have hard rubber, the tread blocks aren't as deep, etc....
I say get some winter tires on dedicated rims - go to Thetirerack.com and pick them out. It will cost you around $1,100 or so, but that's probably cheaper than selling your car and buying a new one. Obviously, when there is snow, you need to drive with more caution, but the winter tires will make an enormous improvement in your traction. If you want, you could probably put some extra weight in the trunk, but that won't do much if you don't change your tires.
C.