I'd wager that the lion's share, if not all, of that growth in total, are among people speaking Spanish. Something over 10 million illegals are 99% Roman Catholic.
You may be partially right about growth among evangelical protestants being ex Roman Catholics. Every evangelical Presbyterian church I've attended has a large share of converted Catholics. I use the word "converted" carefully, as most I've talked with have a born again experience--something they never had in the Roman church. Typically Roman Catholics I know struggle with guilt...as their religious background gives them firm moral standards, which they know they don't measure up to. This "lack of self-worth" (guilt) pushes them to know Christ as a personal Savior--something they never understood from Rome.
In the Protestant "conversions" to Roman Catholicism, I cannot say I've heard of people becoming "born again" hence, it seems "conversion" is the wrong word. If you are a believing Christian already, and then change your church, and even many of your beliefs, finding a "home" so to speak, "conversion" is not the most accurate term to use.
>> I'd wager that the lion's share, if not all, of that growth in total, are among people speaking Spanish. Something over 10 million illegals are 99% Roman Catholic. <<
Be careful not to cite presumptions as fact! Less than 50% of illegal immigrants from Mexico were actively Catholic when they left. Those that were often become separeted from their faith shortly after getting here. Naturally, those with a strong church community and family ties are less likely to immigrate illegally, those who do immigrate illegally rarely like to register with a "gringo" organization like a mainstream church, and the poor often levitate to towards the more dramatic and less demanding churches. I lived for a few years in an area that was 80% Mexican, and you could count the Hispanic surnames registered with the Church on one hand. (Admittedly, however, this was Boston Archdiocese, where, of course, the Church has been infiltrated badly by Satan.)
As for your care about using the word "conversion," I appreciate it. I'd actually have no problems with the word when used to describe Catholics becoming Protestant, since presumably there was something lacking in their faith when they left the Catholic Church; I hope you'll neither have problems with the word conversion to Catholicism either, if I've used it for a different purpose: The switch to Catholicism usually is a very profound change for many who make it.
I'm glad most of us are past saying such-and-such a group isn't Christian, but Catholicism is so radically different from many other denominations, the switch does require a profound change in faith, whereas one may shift among certain Protestant churches without changing their basic beliefs very much. But your point is well taken: they always have been Christian, and remain Christian, so a different word is perhaps suitable to contrast simply becoming Catholic from Protestant as opposed to becoming Christian from non-believer.
The Hispanics coming in to the US are by no means all Roman Catholic. Quite a few are so poorly taught they don't know what they are (fruit of the Mexican Revolution), and many join other church bodies very quickly.
Most of the Hispanics I work with are no longer Roman Catholic, and it could be argued that they weren't to begin with (do to never being confirmed, etc.)