On the question of the text, there are no variants offered in my UBS 3 and no indications of any reason to think this was added at a later date in any of my commentaries. So I can find no reason to think this phrase was not original to the text.
If the question is to whether this phrase intended to teach univsersalims, there seem to be two options. First, many seem to interpret sotor (savior) here not in the sense of eternal salvation but of general salvation. It is used here in the same sense as when Jesus is refereed to as "Savior (Sotor) of the world" in other places (see Calvin, Hendriksen & Kistemaker, others). The other option is to argue that malista (especially) here is not highlighting a group within the whole but identifying the nature of the whole. So it might be translated "who is the Savior of all people, [namely] those who believe". There are a number of other uses of malista in the NT that support this second explanation. In Acts 25:26 and 26:3 it is used in this sense. However, in the pastoral epistles it seems to be used in the former sense of highlighting something within a whole. So the Greek usages can support either option, but the former seems preferable because of its usage in the pastorals.