Is the usage of "extreme power" correct?
To my ears, the phrase "extreme power" is not normally used to connote physical strength.
I think most native speakers would say one of the following:
He was very strong...
or
He had incredible/great/unimaginable/etc. strength...
In some circumstances, you might be able to get away with saying "He was extremely powerful", but that generally references a superhuman (i.e., comic book) strength.