For a plane that size..(12.5 lbs) I would go with two 6S packs.

A fully charged 6S pack (4.2 x 6) will have a voltage of 25 vts. So you will be starting out with 50 volts...exactly what the ESC is rated at.

A minute later the voltage will likely be 45 volts...hopefully it will hold at this value for several minutes.

If you prop for an amp draw of 45 amps, the system will see 2000 watts which represents about 160 watts per lb which will fly that plane in an aerobatic mode.

You will have to play with different props to obtain the required amp draw.

A 5000mah pack passing a 45 amp draw only represents a 9C discharge (45/5=9) so if the packs are rated at 25C there should be no appreciable heat generated in the pack. (Tip: always buy the highest C rated packs you can justify...high C = low internal R)

Granted the 6S 5000 packs are going to raise the overall weight of the model somewhat, so some of these figures will be changed.

..good luck..!

Bill