The Myrmidon may have decent normal speed, but it's afterburner is dog slow.
Also you underestimate the Perseus' handling advantage. It is huge. The Myrmidon is a very clumsy craft compared to the Perseus. Its enormous target profile from every angle makes it extremely easy to hit. It can't carry the Harpoon, which is a huge demerit on a fighter (Hornets can't track fighters well, Tornadoes take up too much room, Trebuchets aren't designed for dogfighting, and Rockeyes are terrible in general). It is completely useless if you actually take three different secondaries because you won't have enough of the first two.
As for dogfighting, dogfighting is pretty much the one thing that you can count on doing in every single mission. Unless you're flying a heavy bomber or doing a stealth recon like Playing Judas, you will be dogfighting somebody at some point. The Perseus has an enormous edge at dogfighting, it gives up next to nothing in secondaries (the Myrmidon's three secondary banks might as well be two unless you enjoy spending half the mission under a support ship), it has more than enough firepower (especially with Kaysers--you don't need more than two Kaysers, and they drain too much energy for any craft except the Ares to handle four), it has nearly as much durability, and it can avoid being hit much more easily.
The Perseus is an amazingly well-balanced craft and is far more deserving of "space superiority" than the Myrmidon. I hated the Myrmidon when I first flew it, and I still hate it now. It's an ungainly, ill-conceived, ugly, bulky pig of a fighter. Its various disadvantages outweigh the value of another couple of gun points.