I have just read a FAQ regarding SGT on makita.be, here's what comes out:
1) When asked about the competition offering 9ah and 12ah battery packs, they answer that 18v 12ah batteries would be technically possible, but heavier, bigger, slower to charge and more expensive than their equivalent XGT counterpart. They are talking about 0,7 and 1 kilogram for their 2.5ah and 4ah version respectively. The 2.5ah can be fully charged in 28 minutes.
According to them, their 40v 2.5ah weighs just a bit more than their 18v 5ah battery and less than any 18v / 9ah option currently on the market.
2) The XGT charger will require an adapter to charge LXT batteries.
3) They will keep releasing new LXT 36 volts tools, they are aware that a lot of customers are already equipped with 18 volt batteries and definitely can't ignore this fact. They still think that XGT 40v would nicely complement LXT for the most demanding tools.
original article (in french )