Right now, I think there is no need to replace the arms with metal ones.
In similar conditions, same vehicle running in similar way requires similar power.
So a 5000 mah 3S Lipo will give almost twice as much run time as a 2500 mah 3S Lipo (pack weight difference may be considered for exact ratio)
Similarly, You may calculate ratios between 2200 & 3300 mah batteries.
3S vs 2S...well if you use 3S 'judiciously' with lower gearing, then you will get more run time / less heat for same capacity of Lipo pack. Otherwise 3S means more rpm, heat & serious wear and tear in the drive train.
Now if you want to replace a 5000 mah 3S battery with a 2S one for similar run time, we have to roughly calculate the mah of the 2S Lipo required to do so.
Let us see:
Your vehicle at a 'W' weight takes 'F' amount of force from 'T' amount of torque from the motor to move at an average speed 'S' against friction & air drag etc for given weight and shape of the vehicle. To this, the motor has to 'work' against the surface friction, drive line friction, air drag etc.
For a given vehicle, this work done per unit time, power 'P' is same for same conditions.
Case 1 - 5000 mah 3S Lipo:
Power required = P (as above)
Power = Volt x Amps,
P = 11.1 x I1
Case 2 - 'Y' mah 2S Lipo:
Power required = P
P = 7.4 x I2
So,
I1 / I2 = 7.4 / 11.1 (Notice I1 < I2 so less heat!)
But I1 is current drawn per unit time from 5000 mah capacity of 3S Lipo
Hence, the ratio I1 / I2 gives us the ratio of mah1 / mah2 i.e 5000 mah / Y mah
Therefore, 5000 / Y = 7.4 / 11.1
or, Y = (5000 x 11.1) / 7.4 = 7500 mah or 7.5 A Lipo pack!!
In this way you can choose 3S / 2S or can identify mah to match your run time as per your requirement.
Please correct me if I am wrong somewhere & sorry for this long boring post as I am quite bored here in office
