I've known folks to go to Santa Croce because there are enough English-speakers on the faculty there that they are likely to take most of their exams in English.  I don't know that it follows that if someone takes classes or earns a degree from Santa Croce that she or he necessarily buys into Opus Dei spirituality.

It's important to remember that seminary formation in Rome is very structurally different than seminary formation in the United States.  Bishop Barres would have lived at, and therefore got his pastoral and spiritual formation and the North American College.  Santa Croce would have only been responsible for his  intellectual formation.  

Now some news reports suggest that Bishop Barres is a member of Opus Dei, but frankly, I doubt it.  his Curriculum Vitae does not seem to square with Opus Dei ministry.  It is possible that he is a member of the Priestly Society of the Holy Cross, a group of diocesan priests who use Opus Dei spirituality, but are not members of Opus Dei.