I just have the standard suspension. The AVS suspension option sounds pretty cool, but could'nt find many used 05's around here with that option. I want to keep my ride relatively soft as that is one of the things that I like about the lexus - (the soft, comfortable ride with minimal road noise and good isolation from bumps), but then I think the 17 inchers should give a bit better feel for cornering and handling, which I agree is a bit lackluster. Anyway, not much I can do about it unless I buy a set of smaller 16 inch rims, which I wont do unless it would make a major difference.
It won't make a major difference. Just stick with tires for your OE 17" alloys....you'll be fine. :)
The only thing I would suggest is if you do decide to get a set of winter tires down the road, get a set of Lexus 16" (2002 - 2006 ES) alloys from an autowrecker or salvage yard for dirt cheap & just install the winter rubber on them.