Hi nikolo
I cant see a way of altering the line spacing in the subtitle room.
However, you do have the option in "TITLE DESIGNER" you may find that you could use ordinary "TEXT" instead of a subtitle.
To open the Title designer, drag a text box down to the timeline and double click on it.
If there is a way to alter the line spacing in subtitles someone else will let us know
Panny TM10, GH2, GH4,