If you want to get interview slot from top tier MC firms, improve your GMAT to 750+ will increase your chance much. But remember, the interview is the key.
If you are in a top B-school and you want go back CHina, then it's fine having <750 GMAT, you will get interview slot. Again, interview will decide.
So, if you have time, I would suggest you do GMAT again. But if don't, then you are still fine.