If you do all the NBME Forms = 1400 questions. This is certainly a good chunk of questions pool.
NBME Forms if done properly
and online will certainly give you an idea about your predicted performance
on the real exam.
However, I don't think they'll increase your performance. For a simple reason, we don't know the answers and there are no explanations. Therefore, it's not a teaching pool and unlikely to improve your knowledge fund.