Yes, the factory empty grommet on the firewall almost directly behind the "go" pedal" is the best location to bring the power cable from the battery.
I am guessing you are going to locate said sub and amp in the trunk?
If so, then you will need to get the power cable back there, as well as determine how you are going to get signal.
I would run the power cable over to the driver side sill, and then down the sill trim to get it to the back and into the trunk. You'll be pulling off much of the lower trim when you do this. Not a big deal, but it takes some time to be thorough and not break anything. A Bently manual and its pics are a great asset in this endeavor.
Once you get the power back there, you will run a ground wire from somewhere on the frame, as short as possible, to the amp. when you get all that trim off, you may find a threaded stud or two that may work for your application (low power, higher gauge wire) with some sanding, or you may wish to drill and sand a location somewhere on the sheet metal (higher power, lower gauge wire).
Lastly, you have to get signal back there. You may be pulling this from speakers, monsoon amp, or direct from an after market head. If the latter, be sure to run it down the OTHER (passenger?) sill so its not running parallel with a power cable for a distance.
Good Luck!