The nice thing of starting with say SL as base is you can have more than90% of the build as SL and have a few extra odd ball software that is not in entropy + you know it works.
Going from say gentoo stage 3 hell or funtoo stage3 you end up with lots of issues most people can not fix or with funtoo you are past SL packages wise "version numbers" and then have to add SL overlay and go back to the future + gentoo / funtoo does not look that nice + does not have the support / testing and fixes so you end up a island on you own problems wise unless you are hardcore old school gentoo'er and SL is just a kick start and don't care too much just need the speed convenience of SL as base to get where you are going.
I am sure there has been a few guy's doing 20 - 40 hours of compiling and then they find out 90% of it was a waist and have to start over or if lucky can use the bins they build if they did . LOL been there done that.
The thing is you have a choice of SL or Calculate or systemrescue and SL is the best to start from if doing something like this. SystemR has a few bugs in the 64bit kernel and instead of just changing from the working 32bit the 32 -> 64 bit setting for the kernel they left out several options so in vmware you can not even see the actual virtualhd's that is there and building on this = error's / have to redo and eventually figure this out if not on a vmware but in live / installed then what else could go wrong using the others.
SL you know it works / solid base. If serious you can make your own kernel or copy and paste + tweak and then build but most folks will just genkernel all and good / bad / defects get transferred and then finally give up as Gentoo is too hard and go back to other linux distro's defeated. That is human nature.
And you know how much fun the kernel options can be and sensitive. The learnign curve and amount of learning in gentoo is too much for most folks.
This is a safe way to learn gentoo using SL as base and grow as you need too.