Has anyone tried using narrower tires for winter? It should cut through snow & slush better but I wonder if 225 could be too narrow especially for an RX450h AWD which weighs about 4650lbs.

For contrast, the Toyota highlander is about 4500lbs and uses 245 width tires stock.

Tire load index rating for 225 is 100 vs 235 @ around 102 so there doesn't seem to be much of a difference in that aspect.

 

Do you have 18 or 19 inch wheels.
You would need a second set of wheels as you do not want to pay over $100 twice a year to swap tires around, with all the chances for tire or wheel damage that may cause.
I would keep to the stock tire size and just buy another set of wheels in 18 inch if you have 19 inch and a set of dedicated Winter Tires. www.tirerack.com can help you out with selecting a good winter tire.
There are lots of used wheels on eBay and note that the Highlander, Venza, Camry and ES350 wheels will also fit as they are on the same chassis/platform as the RX. Usually they are more readily available and cheaper.

 

I stopped seasonally switching tire years ago. Last car I did it on was an 87 S-10 Blazer. You won't notice any difference by going from a 235 to 225, there just is not that much of a difference.

I used to go from 235 to 205 which worked on slush, but, for how I like the thing to handle. seemed worse on snow. Personally, I liked the 31"x10.5" tires on my last ZR2 Blazer for all seasons and and off roading, and would still own one if they had kept making them.