No, I do not believe prayers for the dead will have an affect on their salvation. I will use an example. In 2 Samuel 12, David's son is dying. David prays, and weeps, and begs God through prayers and fasting to save his son. But, when his son dies, he stops these things. The servants are confused by this. So, he explains that while his son was alive, he begged God to keep him alive, but once his son had died, God's decision was made, and there was no use to fast and weep over something that's already been decided. I know these verses are talking about a physical death. But, I don't think that spiritual death and physical death are unrelated. I believe spiritual death is the same, we can pray and fast and beg God to save someone, that he would change their hearts to accept him. But, once the decision is made and over, they cannot go back and change it. I don't see anywhere in scripture that would support the opposite. Even Jesus, when he prayed over Lazarus, did not say he was dead.
But, if someone has scripture to point to the contrary, I'd be more than happy to look at it