I would not probably run more than 750rms total, Not without an extra battery anyway. The big three upgrade is also a good option. Big 3 is replacing the wire for your charging system. Battery + to Alternator, Battery - to Chassis, and Motor to Chassis. Usually wanna upgrade to 1/0 and will help move electrons so your car can better provide electricity to said audio.
ELD in the Honders is a pain, because the car stops charging to save gas. You can offset this by turning on your blower, (hot or cold), turning your lights on, and driving over 65mph. You can bypass it also, but I dont think you want to do that assuming its still under warranty.
If your Fit is 09+, when you remove the radio, you will have to pop a panel out just above your lower drink holders and remove a screw before the radio pops all the way out.
The tweeters will pop out easy up top, I suggest cutting holes in the factory plastic cups or poking a hole through and running wire up to new ones on top (great factory location, and even better if you have the tweeters facing you)
The door panels come off by popping the outer plastic part of the arm rest off, you can see the clips if you look at the underside of the arm rest. The door panel clips attaching it to the actual door are a pain and may break so be careful.
Factory speakers are molded into their own little adapters. I took the factory speakers out and cut the actual cone out, then used the remaining piece for a speaker adapter pod.
Rear speakers I wouldnt even bother with.
In the back you can ground near the cargo loop in the corner near where the seats fold down.
When removing the interior trim to run wire, it is easiest to pull the door seal off or partially off and then pull straight up on the trim panels, they are connected in a weird mannor and this is the easiest way.
There is a rubber grommet behind the dash sorta up to the top on the passenger side which is good for running power wire through, just not when the engine is hot lol.
It doesnt take much to get this car loud either.
I can help with pics if needed