I actually did the same for my 2012 MBP; got an 11ac card, dropped it in, done. Now see ridiculous speeds when on 11ac (at home & in the office). Definitely worth the $40 on eBay for the new card.

OTOH, my laptop is now 4 years old and the 750GB SSD is pretty much full. Frustrated by the lack of new MBP from Apple... friends I have who work there on these things just smile enigmatically and say it'll be worth the wait. Grrr.