What do you mean by optimizations? Your MAKEFLAGS are going to be static based on your CPU. You will set the CPU type when you compile the kernel which will also help. You can customize the kernel for your hardware, but really this will just save time compiling the kernel itself; you won't get a huge performance boost for this. And, you can set USE flags based on software requirements; this will help a little with performance.
Most of these, however, you can accomplish with Arch. The kernel is the one thing I do consistently recompile for my system.
I think the real advantage to Gentoo, as far as having old hardware goes, is it's easy to stick with old versions of some software while staying up to date with others (it's also easy to have multiple versions of the same software installed). With Arch, it's assumed that you stay up to date with everything. If you want to get fancy with Arch, you might have to get your hands dirty on a package-by-package basis.
Sorry, I didn't know that about wakealarm, I thought it was an entirely software thing and that it just depended on the suspend level you are in (S1 vs S3...).