You'd be wrong and underestimating.
MS-13 is probably at least 75% illegal (and thats a conservative number).
You'd be hard pressed to find even one, just one, that was born in this country.
Personally, and I have seen and met several, I think the numbers are much higher then that. They're illegals, but they like to recruit (and or attack, exploit, or victimize) legals. That said, most of their members are either brought over or are other illegals they get over here.
They have a MO that is rapidly changing.
However, just looking them over and how they operate, its hard to see how they could have more then a handfull of legals.
What really ticks me off about MS-13 is their lack of citizenship is a powerful shield against prosecution, the death penalty, etc. We are more likely to deport them than to keep them in prison. They are like the Normans in the Dark Ages. They thrive off going 'on a viking'. Later they carve out private fiefdoms.