BAD : Since he stopped smoking, there has been an extreme improvement in his health.
GOOD : Since he stopped smoking, there has been a dramatic improvement in his health.
BAD : Doctors have to keep up with all the extreme new medical developments.
GOOD : Doctors have to keep up with all the major new medical developments.
Extreme is mainly used to describe situations, behaviour, opinions and beliefs: 'extreme poverty', 'extreme caution', 'extreme views', 'extreme disappointment'.
When you are talking about a change or development, use words such as great/major/important/considerable/dramatic : 'Without these major changes, the company would have collapsed.'