As to you GFCI question. A GFCI outlet will, protect everything downstream of it. That means that if the GFCI is inline ahead of the other outlets (meaning it comes from your circuit breaker box to your GFCI and then out to the other outlets) all will be protected. Though it you didn't wire the house or are not 100% positive about the wiring of the house, I would buy a GFCI tester. After all the non-bathroom outlet could be first in line, and then go to the GFCI in the bathroom (I believe code requires GFCI in the bathroom) and you would not be protected. There are a simple plug in device that will indicate if an outlet is wired correctly, and with the press of a button will trip the GFCI. Its a simple way to be sure you are GFCI protected. Just search GFCI tester on amazon or ask at your local hardware store.
As for you element, I assume that a 1500 watt element and 15 amp circuit means 120 volt. watts/volts gives amps, so you would be pulling 12.5 amps. That will not trip the breaker, but as you mention the two bathroom outlets are on the circuit you might run into some issues. If anyone wants to use the outlets in the bathroom they are left with 2.5*120 watts (300) to play with before the breaker will trip. That isn't enough for a hair dryer or an electric heater. If there are any lights on the circuit that is going to be draw as well, you'll only get a few lights at that level; take it from me Satan has nothing on a woman who just had all the lights go out in a dark bathroom in the middle of a shower.