I have had problems in the past with churning Chase Visa cards and getting the bonus again. In the fine print it says the bonus is only for first time card holders for the specific card. This was the case for the Marriott 20K sign up bonuses but I have not tried it for the SW Visa. The article above for how to churn the Citi AA cards is slightly different in my experiences because Citi never mentions the bonus is a one time thing, rather sign up spend the $750, get 25K points, rinse, repeat, no problem.

As for credit scores, I can only speak for my family but my wife and I have opened and closed over 60 credit cards in the last two years and my score is a 780 and hers is a 790. So it can vary based on a multitude of factors as mentioned above.