My 8 is completely stock...and i do not know very much about them.

Would adding a pulley be a good performance upgrade. If so..what kind?

 

No... it will not.
Start reading through the threads to get an understanding on everything.
Learn to use the search feature as most of your questions have likely been answered MANY times.

Here... I'll get you started with the basics:
https://www.rx8club.com/series-i-aftermarket-performance-modifications-23/new-rx8-owner-faq-1-25-09-a-163824/
https://www.rx8club.com/new-member-forum-197/rx8-faq-new-owners-116484/
https://www.rx8club.com/rx-8-faq-9/faq-update-63747/