Your power will depend on the tuner, going down in compression means you can run more timing and more boost meaning it is possible to get more power.
It is not a necessity though, so is totally up to you. (it makes negligible difference in all reality, but is a good excuse to look at the condition of your engine before you turbo it)
Factory compression is 11.5:1 and I think double stacking factory head gaskets brings it down to 11.1:1 or 11.2:1, so still high!
Edit: According to this http://z4.invisionfree.com/lexusaltezza ... topic=7121
it lowers to 10.5:1 so it is a decent drop.