I have seen the reduced affect of a lower C rating battery like yours, compared to my 50c ones, mainly towards the end of a run.
But it was still way quick enough and may actually of helped control the wheelspin
Think of it like a slightly rich nitro down the straight.
This should not lead to any swelling and I would just use them until you can afford a better quality (and C) battery.
Note if your planning on doing the 1/8th electric Nationals you WILL need hardcase.