@monkey mart said:

The above comment brings up a good point about potentially scoring lower on a future exam. Although school's place greater weight on your highest score, admission officers might still be concerned if you re-took and scored lower.

However, scholarship $$$ is also something to consider. With a diagnostic in the mid 150s, you could have a great shot at a higher official score with additional studying, leading to potentially higher scholarship offers. It ultimately comes down to what your goals / priorities are and whether additional studying for a (potentially) higher score is worth it to you.

If you were applying to schools with higher median LSATs, it might make more sense to aim for a higher score. But for this school, your current score is within the range of the students they admit.