Overdoing it does seem quite likely - if I didn't know and was trying to do a test I could easily see myself just enabling all the optimizations and moving on.
I would as well. My understanding is that SystemD does more parallelization/magic to decrease boot time, but is a bigger system to try to comprehend and has more surface in general.
I believe parallelization is a configurable option on OpenRC though, I'll add that test to the list to try some time.
Boot time crossed my mind, too, but Gentoo is happy to run with systemd so that's not really "Gentoo vs Ubuntu" so much as "this exact variant of Gentoo vs default Ubuntu" which is kind of fair. (Assuming, of course, that OpenRC is slower to boot than systemd, which would also be an interesting benchmark)