I don't believe any Christian has the right to undermine or minimize "the signs of an apostle" in this matter.
The reason Paul specifically calls them "the signs of an apostle" is because God and Christ wanted the world to know that they had given these men miracle-working powers to prove that they were truly apostle-prophets and that the Gospel was truly from God.
Miracle-working powers through prophets were not seen within Israel on a continuous basis. Moses, Elijah, and Elisha stand out. But Jesus of Nazareth would show Israel through His miracles that He was not only the Prophet whose coming was prophesied by Moses, but He was also their true Messiah -- "a Man approved of God". Hence Peter says in Acts 2:22: Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:
What is noteworthy in the New Testament is that although the apostles must have also performed probably hundreds of miracles, the Holy Spirit saw fit to only present a few of them for the apostles -- but not for Christ. This would indicate that the reader's attention must always be on Christ (and this was also the goal of the apostles).
With the passing of the apostles the sign gifts as well as the spiritual gifts of apostle and prophet ceased. All those who claimed these gifts later on proved themselves to be false (e.g. Joseph Smith).