No real need to by from the dealer, however getting the correct spec PS2 will improve the ride/handling of the car.
No all PS2 are created equal. See specs below pulled from Tire Rack. My guess is the extra load rating is a result of stiffer side wall.
245/40ZR18 (93Y) SL BMW 220 AA A 1433 lbs.
245/40ZR18 (97Y) XL AO (Audi) 220 AA A 1609 lbs.
I know when purchasing off road tires for my land rover, Going with the lowest load capacity tire (while still being higher than the Max GVW) results in a much smoother ride off highway. Basically extra flexibility in the tire adds to the overall suspension of the truck.