Speed Ratings from Y to W for IS350
 

The car came with Dunlop SP Sport Maxx A summer tires, but they wear fast. These tires are Y-rated.

I am considering Bridgestone RE960 Pole Position which are W-rated.

Is there any problem going down from Y to W speed ratings? What are the problems that can occur?

The other choice is the Michelin Pilot Sport A/S Plus (Y-rated).

Any input is greatly appreciated.

I'd be cautious about any A/S tire on the IS350. You will spin a lot / have TC kick in a lot in first gear, and drift under throttle even through 2nd. I have some Eagle F1 A/S on my car now, and I absolutely hate them.

That fast wear means you are getting good grip. =)