Can I Leave Thailand and Return on a METV Before Its Expiration?

As long as you ENTER on or before the expiration date of the visa itself you will get another 60 day entry AND you can extend that by 30 more days at the immigration office for 1900baht.

If you wanted to leave and come back during those 90 days you would need to buy a re-entry permit at either the immigration office or at the airport (if where you're flying out of sells them) before you flew out.

it will not add any days to your stay but it will allow you to leave, come back and get stamped in for the same expiration date.