Focus fire on the mid-impact ARs like Silimar's actually worsens its TtK by a huge amount. Also, FF doesn't go well with persistence. You'd hit higher accuracy quicker firing at full speed. Admittedly, the other perk options in focused fire's slot aren't too attractive as well. Icarus might be useful if you're a bumper jumper, and Hot Swap is a deceptively nice perk for accuracy. The rest are pretty shitty.