1515 2Y on 6S???

[Finnster]

You are keeping the same total energy in batts, so you cannot expect a big % gain in runtime on such a swap (considering the 1Y is quite eff in the first place.)

To increase runtime, you will have to increase batt size (and weight.) The energy has to come from somewhere. You may get a min or so at most, but can be easily negated by driving style or pile up of mud/dirt on the car.