"This is a faithful saying and worthy of all acceptance, that Christ Jesus came into the world to save sinners, of whom I am chief." 1 Timothy 1:15
Not was, am.
I'm not so sure Paul was expressing that he considered himself the foremost of sinners at the time of his writing. He wrote elsewhere that he "didn't know anything against himself", that is, that he had a clear conscience. Perhaps he meant that his sins were worse then anyone's? Though he didn't continue to commit those sins, such as persecuting the Christians.
Romans 7:17-20 KJV
17) Now then it is no more I that do it, but sin that dwelleth in me.
18) For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.
19) For the good that I would I do not: but the evil which I would not, that I do.
20) Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
Here Paul twice tells us that now it is no longer I that commit sins, but sin that lives in me, specifically in my flesh. Until our minds are renewed we think according to our flesh, and commit those same sins. As our minds are renewed, we come to think according to the new creation we've become, being born from God, and do those righteous acts which come from the Spirit.
Much love!