September 2, 2015 2:55 AM CEST
Sorry, did not understand you had already modded the PF battery.
Old 9V-type cable isolation is very poor, new PF cable isolation is somewhat better... but when using a few Amps even the PF metal contacts tend to get to hot, so if you don't burn the cable isolation you melt the plug.
If you really want more power, consider using RC car/plane batteries. I used a 2S 7.4V LiCoO2 1500 mAh battery for my early SBrick stress tests, costs 1/4 of a LEGO LiPo and lasts 2x more (but you also need a charger). It has 25C capacity so in theory you can get 37.5 A (play safe, please).