I wanted to thank you as well!
While I didn't really replicate your testing, when I read this I hopped into mock battle with SM!Felix, equipped with nothing but boost Critical and then boost Critical + Apex Sword. I also attached the Myrm and a single regular attack. So pretty consistently he was doing ~350 on a crit w/o Apex Sword and then ~300 with Apex Sword. (The numbers aren't exact.) So this seems to match your findings.
As an aside, I then equipped boost Critical + Luna, just to see how the numbers changed, and I was getting 3000 with a crit! I thought Luna would be good but that's a shit ton.