Flap 25 takeoff is permitted in the -700, but not -800.
I've done several Flap 25 takeoffs in the -800. From memory I remember at one airport in calm wind, one end with distant terrain, the other without significant terrain, the Flap 25 RTOW was significantly less on the end with terrain than the Flap 5 RTOW (with the 25 being obstacle limit and the 5 rtow being field length limited), but on the end without terrain Flap 25 was around 500kg more than Flap 5.