I was trying to see if we have options to go to 8S setup on our birds. but what i have in mind becomes heavy and does not handle the Amps.
You need to think in terms of watts, not amps. For two systems of equal efficiency, an 8S setup should be about the same weight as a 6S setup, because as the voltage goes up by a factor of 4/3, the current would go down by the same factor. The actual power required out of the cells, and the total energy stored in the cells, would remain the same.
So, for instance, a 64 ounce plane with a reasonably efficient system is going to use around 45 watt-hours. Using the "75% rule" you'd want a battery pack that's rated for 60 watt-hours (this is conservative; mosts people with good-quality packs shave this figure). Getting absurd, you could theoretically do this with a single 16200mA-h cell. Or you could go with four cells of around 4000mA-h, five cells of around 3300mA-h, or eight cells of around 2000mA-h.
Money for new chargers aside*, the problem becomes one of efficiency. The greater the mismatch between the battery voltage and the voltage the motor needs to run, the more switching losses the system incurs. You want to more or less match the motor kV with the desired RPM and full battery pack voltage, to avoid switching losses in the system. You also want to match the ESC's capabilities to your projected maximum current (lower with more cells) and maximum voltage (higher with more cells). So a 5S system that works perfectly well may need a new ESC to work at all on 8S, and it may not run efficiently unless you find a same-size motor that has a lower kV to match the higher battery voltage.
* Lots of RC folks who fly 8S or 10S fly on two 4S or 5S packs, charged in parallel and flown in series -- but that takes more connectors in the plane, which incurs a weight penalty.