My personal experience is that reading in general (intensive, extensive or whatnot) improves your reading skills drastically, increases your vocabulary noticeably but per se won't help to to write and especially to speak.
Talking specifically of intensive reading - let's give some definitions for the starters. According to British Council:
Intensive reading involves learners reading in detail with specific
learning aims and tasks. It can be compared with extensive reading,
which involves learners reading texts for enjoyment and to develop
general reading skills.
Example The learners read a short text and put events from it into
Again, I can talk only about my personal experience here, I'm not aware of any studies, but the only difference between extensive reading and intensive reading for me was that if there are written tasks (based on what I've just read) - it helps to some, quite limited, extent to improve writing skills.