Right, you are using G.dmt, PPPoA. Use MTU 1500. (1478 for PPPoA is a very slight performance tweak, slightly more ATM-efficient, as has already been said, I would have to do the arithmetic properly to work out exactly how much. But if I were you, I would not go that way, stick to 1500 as that way you won't have any problems, compatibility over minuscule speed increases.)
Can you confirm you are on a 20CN exchange, not 21CN? Would be worth ticking the ADSL2 tick box _in addition to_ G.DMT which you have ticked already, just in case ADSL2 works. It won't be available if you are still on a 20CN exchange.