How efficient is a Peltier TEC1-12706 and TEC1-12715
We'll start with the easy one first: Do not try to cool a room with TECs. It simply will not work. Forget it. Don't even think about it. Cooling loads for room air conditioning will run in the hundreds of watts.

OK, with that out of the way. Yes, hot side temperature will depend on ambient. Assuming you're using a heat sink of some sort, the best hot side temperature you can get will be ambient. And since, in order to shed heat, the heat sink has to get above ambient, that's a lower limit. Typically, a TEC heat sink will be cooled via forced air, which reduces the heat sink temperature.

Second, the temperature differential will be closely linked to the amount of heat you're trying to remove. The 65 C number you've run across is for essentially no heat flow. Any increase in heat removed will reduce the temperature difference.

I suggest you play around with a TEC calculator such as this one. Consider the following starting point. 

1) Ambient temperature is 40 C
2) Desired room temperature is 20 C
3) the heat sink is maintained at 60 C (Note: this is roughly the "Ouch!" point where you can't keep your finger on it.)
4) The heat load is 500 watts. For instance, that's the heat put out by 5 resting adults. Unless the room is very well insulated, you might get that amount of power just from the external temperature coming through the walls and air leakage.

Run the calculator. See Qmax? A 12706 has a Qmax of about 60 watts, so you'd need about 32 modules. Each one would be run at about 3 amps. From the 12706 data sheet you'd want to drive your modules at about 9 volts. Total dissipation would be about 860 watts. And before you think that's not unreasonable, you'll need to price out a heat sink which will mount 32 modules and provide a thermal impedance of .023 deg C/watt, which is the temperature difference between hot side and ambient (60 - 40) divided by the dissipation (860 watts) at 860 watts total. Trust me, this is a monster. 

Get an air conditioner.