You obviously have the grades so that doesnt matter too much. SM is right that 90+% of the time grades will not matter, with the exception of your first job after college and even then it sometimes doesnt matter. You are also right that good grades will definitely help you get into a better school.
Now the question of the matter becomes with joining PTK help you get into a better school? That I cant really answer with the exception of personal experience. With my grades and letters of recommendation I had zero trouble getting into the likes of SMU, TCU and UT Austin without it.