Is the validity period for METV the same as SETV, and how can you maximize stay duration?

You need to also get the 6 month METV as close to the date you're comin' to thailand as you can. Because it's valid for 6 months from the date it's ISSUED. <- Meaning if you get it too early you're burning the validity of the visa not being here.

It also doesn't matter if you extend any of the entries by 30 days or not, EXCEPT the very last one.

you exit/re-enter the country just prior to the "enter before" date on the visa to get another 60 day stamp and then you extend that one by 30 days.

No matter how you do it you can't get more than 9 months out of it.

60+60+60+60+30=270

a Single Entry Tourist Visa can be used once to enter Thailand, it is valid for 90 days after putting it in your passport and you get 60 days Admitted to stay in Thailand, you can extend this once with 30 days at local Immigration. a Multiple Entry Tourist Visa is valid for 6 months starting at the moment putting it in your passport. From that day you can enter Thailand as many times as you like and will get on every entrance 60 days of Admitted to Stay, which you can extend once with 30 days at Immigration Office. If you like to make the most out this visa you leave and re-enter Thailand on the enter before/valid until date stamped on your visa, if you extend this Admitted to Stay period with 30 days you can get almost 270 days inside Thailand out of this visa.