DON'T DO IT!!! They aren't the same mah! The 5000mah will die before the 5200mah and you will go below 3V/cell on the 5000's before the 5200 hit the cut off, you risk destroying your lipos!
Just set the cut-off to a higher and safe number like 3.4v and it will be fine. That small 200mah difference isn't a big deal.
If you want to risk your lipos, esc, truck, and possibly life thats up to you.
Don't be so over-dramatic. Batteries in series see the same current. They will discharge at the same rate. If you run the batteries down to 0 mah left you were running them dangerously low to begin with.
Nice. Mine made a huge difference. My rear rod ends were worn so bad the rear tires would point outward ever time I hit the gas. That problem is completely gone now.