I was speaking to the maximum power available to a vanilla wizard. Take Harry. Say he was born with a gas tank of 50 gallons. The better he gets by practicing and learning, the more bang for the buck. If it took .5 gallons of gas to power a fire spell in year one, by year ten, the same fire spell with the same effect might cost .2 gallons. This is an improvement in output by efficiency. But it didn't change the size of his gas tank. To increase the size of the tank, you strap on additional tanks. The increases your max available power. But how much of an increase you see is is a function of how much you started with.
So Fix and Harry both get Mantles. Fix's gas tank before the mantle is 5 gallons. Harry's is 50. The Mantle is 50 gallons. So Fix's power increases by a factor of 5 to 55. Harry's by a factor of 2 to 100.
Soul Fire and Hell fire seem to work slightly differently. Hell Fire appears to be a bigger increase in efficiency, but doesn't increase the size of the tank. Soul Fire appears to do both. With a caveat. The soul keeps you alive. You can use it, but when that tank is empty you die, since you have used your life force. This favors an older wizard with more practice and a better technique. He is less likely to overuse it.
This is my head canon and may not reflect the truth as JB sees it, and YMMV.
