When setting up the gentoo system, make sure you read up on optimizations! Remember that everything in Gentoo has to be hand-tuned. It's like a hand-crafted lambourghini...screaming fast and customized to hell, but it's a labour of love. Check out options to put in your make.conf file (like compiling with -O3 and -funroll_loops and other optimizations), and put in every USE flag that you can to make things fair (since I believe redhat comes with the works). If you don't know what these mean, there's lots of documentation on the gentoo site. It's one of the most well documented distros I've seen.
You'll also manually have to start hdparm. There's lots of documentation available for it and I'm pretty sure it's in gentoo's portage system. If you're not familiar with it, it's a program that dramatically improves memory access time in linux by using things like DMA. I know RedHat installs it with some default settings, so you might want to check out what those are with "hdparm /dev/hdX" and duplicate them on gentoo. Just trying to level the balance here
For speed it's difficult to say. You have to be certain to try a wide range because RedHat tampers with a lot of its software. For example, I believe the version of gcc they include with their distro has redhat-specific extensions, as does their X server, and I know they've done a lot of work on their OpenOffice.org.
Anyway man let us know the results!