Mentioning Subject GPA on SOP
I think your SOP should focus on that, not your past record. The committee will have access to your grades and will focus most on the subject grades in any case. There is no need to waste words on things that can better support your candidacy. Yes, you are proud of your past work, but your CV already shows that. 

Write about what you want to do in the future, how your interests and skills align with that purpose. If necessary, talk about how you fit well into the program you hope to join. 

Make your SOP work to complement your CV, not to repeat it.