I purchased a 2017 GS350 Sport with 19" 235 /40 R19 tires. They are rough riding and loud. I want to change the tires to an 18" rim and tire and something with a taller sidewall. I was told by the dealer it might mess up the suspension and handling. My previous 2007 GS 350 had 225/50 18" tires and had a much better ride.

Your wheels must be 8 inch so you could probably go to 245/45 19's and get an improvement without having to buy new wheels, some of the noise and roughness could be caused by the type and condition of the tires.

Sorry if I missed your question...

@rgallucci You will not mess up your suspension by downsizing your wheel size - as long as you keep the same overall rolling diameter. For 19's on the 4GS, Lexus offered stock 235/40, for 18's the stock size is 235/45 and for 17's the stock size is 225/50.

I'd explore getting a different tire brand or category of tire at your current 19" wheel size - many folks are very happy with the Pirelli Cinturato P7 All Season tires as a compromise between handling and comfort.

You can do 18's, it will certainly look less sporty but that's the compromise. My 2013 Lux has stock 18x8 +45 wheels and 235/45R18 tires. You can go that route (or trade/buy from someone wanting to upgrade to 19" OEM, for instance.)

You could also try to make the absolute most out of the wheels you have, by optimizing with a more quiet and comfortable tire in the same size you have, or push it to a 245/40R19 like many have done here.

Thanks for the info. I fully intend to change the rims since they are lifeless and I want to be prepared when it comes time for new tires.
.

This is mine, just gives you an idea. Some other members have taken these exact wheels and spaced them out with 15mm front and 20mm rear bolt-on spacers, definitely makes it look better/more flush.