A kit will only unbalalance things if the total cost of the PC, including the kit, exceeds the total budget for PC's in the campaign world. Once should compare the avergae fireball with th evalue of an avergae hit with a weapon. Since many PC's will make a save, this will actually be an avergae of 9 points of damage. The lesson is that the average affect should not determine the cost, not the extremes. Well if you have a way of rewarding those who do not max out their budget for powers, then it is no problem.
Players can also taylor make classes in a systematic way and still keep things balanced.
We awarded each a certain cost. We then gave a total budget to players. The higher the base, the slower PC's advance in levels. Weak PC's zoom up the levels. It all balances out in a systematic manner.