Yes it has the capacity do do the PWM at 25kHz. The MOSFET will definately be fast enough do do what you want, some of them can switch in the MHz range. A schematic would be much better, pictures are hard to figure out as are frizzy drawings. Here is a simple circuit from Stackexchange.com. This circuit is used a lot, R2 is connected to the GPIO as it should be. If it were connected to the gate it acts as a voltage divider decreasing the voltage to the gate of the MOSFET. It is very possible you can change the PWM frequency of your existing unit by changing a capacitor internally to it. The ones I have looked at used a NE555 as the PWM controller with a cap and resistor controlling the PWM frequency.
