My first long term campaign, run between 1984-87, ran our group of first edition characters from roughly 4-6th level to 14th-16th. A few of the characters were multi-classed and a lot of house rules were involved.
Many years and games later I ran a campaign set in the same world that went from 1st to 12th or so using 3rd edition (and a lot of house rules...its me after all, I equate RAW with unfinished

). While this may not seem especially powerful but many of the characters were nearly superheroic in ability and the campaign was international and interdimensional in scope.
AD