Count me as happy with a booster fan. I have an upstairs bedroom that
is at the far end of the duct run and only served by small round ducts
that run from the end of the basement run all the way upstairs. I
added a booster fan near the end of the duct run in the basement and it
made a noticeable difference in summer cooling. It didn't entirely
solve it, but it helped and the cost/benefit was worth it.