do a power of good, to

do someone a power of good

 and do someone a heap of good
Rur. to be very good for someone. You should take a vacation. It'd do you a power of good. Just hearing your voice does me a heap of good.
See also: good, of, power
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

do a power of good, to

To effect a great improvement. This somewhat unsophisticated turn of phrase uses “power” in the sense of “much” or “a great deal.” For example, “Those vitamins will do her a power of good.”
See also: of, power
The Dictionary of Clichés by Christine Ammer
See also:
  • do (one) a/the world of good
  • expecations
  • expectation
  • come up to (one's) expectations
  • come up to expectations
  • be good for (someone or something)
  • be good for something
  • do one's heart good, to
  • do (one) a heap of good
  • be good news