Check your owners manual....
For 17 inch wheels, a narrower rear tire is specified for winter tires ... 225/45-17 Use same Porsche wheels in Boxster sizes summer and winter.
You'll note that 18 inch wheels are not listed under snow tires and there would be no size change for 16s.
I have the narrower tires by Vredestein--Wintrac-- to mount on mine soon. Although not N-marked, these were on Porsche's list of approved tires. They are normally priced.
A really reasonable ebay seller for wheels:
https://www.ebay.com/sch/i.html?_dkr=1&iconV2Request=true&_blrs=recall_filtering&_ssn=evifenocvt&store_cat=0&store_name=evifenocvt&_oac=1&_nkw=wheels%20rims