How much BTC/MHS/DAY getting from this merged pool?
Over a long enough time you should expect the same BTC/MHS/DAY rate from all pools minus their fees. The difference with mmpool is that hashrate is currently low enough that you may go days without payment and then get a big payment all at once.
Mmpools newly added PPS for BTC is a nice little bonus, bit without even factoring that into my decision for using them, their DGM payment method has to be my favorite. Yes, you won't get a payout daily, but when you do, like @TechByPC said, its a very handsome payout.