I have used Gentoo in this way on many small computers with various kinds of Atom CPUs and with a small DRAM memory and small and slow SSD/HDD.
With multiple computers of a compatible kind, the files from such a chrooted installation compiled on a powerful computer can be copied over all of them and everything will work fine. If the chrooted installation is preserved, it can be updated later with other software packages and all the changes can be propagated with rsync on all other computers.
Linux is not like Windows, which will complain when run on a different computer than that on which it was installed initially.
It's just an unwieldy amount of extra overhead, disk space, and time, which I'd rather avoid, especially for devices I'm not fully committed to maintaining over a long period. I've tried what you mention, it's just never convenient enough to be worth the pain.
My point is that you do not need to setup a binrepo or any other complication like this.
You can install easily Gentoo on a very weak computer, by performing the installation on a typical desktop computer, which may run a different Linux distribution, not necessarily also Gentoo, and then just copying the files.
The Gentoo manual has always included information on how to install Gentoo inside a chrooted environment.
Right, I am sure they intended it to be absurd in an amusing way.
Although, I do somewhat think that working out good optimization flags for cross-platform compiles is a moderately unusual skill, even among people who compile things regularly. Hopefully I caveated that sufficiently, I’m not saying it is a hyper-advanced dark art or anything.
I have in the past set up code on a cluster to just compile on one node for the first run with -mtune=native because I’m lazy!
But to be honest, I haven't looked at binrepos so far. Perhaps your reply is a good reason to.