I think this is some weird game Discount Tire is playing in order to turn you to a different tire they carry. Check with Tire Rack to confirm your load rating but for stock N2 tires it looks like 89W, so the DWS at 91W exceeds Porsche weight rating specs.
https://www.tirerack.com/tires/TireSearchResults.jsp?tireIndex=0&autoMake=Porsche&autoYear=1976&autoModel=911&autoModClar=&width=185%2F&ratio=70&diameter=15&sortCode=37600&skipOver=true&minSpeedRating=V&minLoadRating=S
I have been given this line when shopping track rubber at America's tire in non-stock sizes and after looking up the N2 weight rating, my tire choice exceeded Porsche specs for my car. They lost the sale on both sets of tires to a competitor. I generally like AT service and pricing but I won't play this game. If I choose tires that meet or exceed Porsche N2 weight rating specs, AT can install them for me or watch me walk don the street. We all have choices.