I think Burnett was always planned, assuming there was enough time and DJ to do it. The first two seasons contain the prophetic conversation with Gina in the pilot, Crockett himself making a lot of gripes and remarks about the identity issues the job gives him ("what are you like, really? Flipside of Burnett?" "That... gets a little cloudy sometimes."), and the appearance of Burnett in his "true form" at the end of "Sons & Lovers". Once DJ decided to return during the second season, they had enough time to run the Hackman/Caitlin/Burnett arc over three years.
I have also wondered (and joked with a friend) "is he acting...?" and the answer is partly "no". https://archive.ph/iHjBR (Fixed link)
Apart from saying Crockett's gold watches were pretentious and he wouldn't wear one, I think he largely got to be himself with a tweaked background and accent, and the people in charge of MV were fortunate in having such a strikingly charismatic individual as Sonny.
When he was more obviously acting (Burnett), it was also fascinating to watch, although I think Burnett was drawn from a lot of his experiences and personality as well, as he's gone on to play some other (even worse) bad boys pretty notably.