TireRack doesn't show a 16" wheel available for a 2009 RX350 AWD. That leads me to believe 16" wheels will not provide adequate clearance. Just get an AWD model and pair it with some good on/off-road all-terrain tires: TireRack Tires
Check the Lexus forums. As much as I like TireRack, they often do not have the alternative items that could fit.
I once had an 03 Focus that came with 16's from factory. When looking for a winter setup, the smallest thing listed online was 15's (I wanted to run 14's).
After some running around, I realized the 14 inch steel wheel setup from an early 90's Tempo/Topaz cleared the rotors perfectly.
Strangely, the optional Tempo/Topaz alloy 14 inch wheels did NOT clear the front rotors.