Chinasaur posted a link to some distro reviews yesterday (link). It should give you some idea of what to expect from Gentoo, read the distrowatch article if you want some idea of the current mess in the benchmarking area.
From what I know I do not think you will see much direct benefit from running Gentoo, if you are running with a minimal installation then Gentoo will perform about as well as any other distribution. I'm guessing that you will be using this as a desktop and by optimizing other applications you may see some benefit just because if they don't use the processor as much then DF can take over. However, there are two important points: the current DF client has a pretty wide deviation from the average production and you may not notice the improvement, the other point is that Gentoo does not optimize itself and the installation process is long, if you want to learn about linux this can be pretty interesting and useful, but if you want instant performance look somewhere else.
I think you can reach a good middle ground by compiling your own kernel, by going to 2.5 from 2.4 I managed to get 2.5% improvement with the Phase I client, I haven't had time to start poking things around with the new client yet but I think Dyyryath has something up his sleeve.
Not that I'm really trying to discourage you from using Gentoo, it is a lot of fun, just don't expect your production to double.