If I use a 30-day visa exemption while my METV application is pending, will my approved visa allow me to stay longer without a new entry stamp?

The consensus in the comments indicates that if you travel while your METV (Multiple Entry Tourist Visa) application is pending, it could risk the approval of your visa. If your METV is approved while you are already in Thailand under the 30-day visa exemption, you would need to leave the country and re-enter to activate the METV. Simply receiving the approval while in Thailand does not grant you an extended stay; you will need a new entry stamp to validate your new visa.