Here's a website that shows a chart of final compression ratio. Its a chart plotted with initial compression ratio on one side and psi on the other. Since street engines are limited by pump gas, your limited on how much pressure you can add before you're out of octane. When that happens; Kablooey.
http://www.goodvibesracing.com/Compression%20Ratio.htm
Dropping CR does not remove as much as a turbo gives back. As long as you don't drop below 8:1 or so, you won't notice sluggishness when the turbo is not spooled up, and depending on how well you tuned the turbo it will spool up almost instantly after you mash the pedal anyway.
Just dropping from 10:1 down to 8:1 you might lose 10% hp, but the 10-ish psi you can now blow will increase your power output by about 70%