I have yet to see a port job that didnt add power. However, i have seen polishing jobs that have reduced power.
When you do a polish job, you want to polish like the first 60% of the port. If you polish the port too much, fuel can actully collect and condensate inside the port cause puddling backfires, and erratic lean/rich conditions. Ive only seen this happen a couple of times, but they were all engines that the fuel is injected into the air stream as opposed to directly into the combustion chamber.